diff --git a/app/client/declarations.d.ts b/app/client/declarations.d.ts
index 7b9c9cdf..0fa0fe36 100644
--- a/app/client/declarations.d.ts
+++ b/app/client/declarations.d.ts
@@ -335,3 +335,7 @@ interface Location {
   // historical accident than an intentional choice.
   reload(forceGet?: boolean): void;
 }
+
+interface JQuery {
+  datepicker(options: unknown): JQuery;
+}
diff --git a/app/client/widgets/DateEditor.js b/app/client/widgets/DateEditor.js
deleted file mode 100644
index d11f4cef..00000000
--- a/app/client/widgets/DateEditor.js
+++ /dev/null
@@ -1,172 +0,0 @@
-/* global $, document */
-const moment = require('moment-timezone');
-const _ = require('underscore');
-const gutil = require('app/common/gutil');
-const commands = require('../components/commands');
-const dispose = require('../lib/dispose');
-const dom = require('../lib/dom');
-const kd = require('../lib/koDom');
-const TextEditor = require('./TextEditor');
-const { parseDate, TWO_DIGIT_YEAR_THRESHOLD } = require('app/common/parseDate');
-
-// DatePicker unfortunately requires an <input> (not <textarea>). But textarea is better for us,
-// because sometimes it's taller than a line, and an <input> looks worse. The following
-// unconsionable hack tricks Datepicker into thinking anything it's attached to is an input.
-// It's more reasonable to just modify boostrap-datepicker, but that has its own downside (with
-// upgrading and minification). This hack, however, is simpler than other workarounds.
-var Datepicker = $.fn.datepicker.Constructor;
-// datepicker.isInput can now be set to anything, but when read, always returns true. Tricksy.
-Object.defineProperty(Datepicker.prototype, 'isInput', {
-  get: function() { return true; },
-  set: function(v) {},
-});
-
-/**
- * DateEditor - Editor for Date type. Includes a dropdown datepicker.
- *  See reference: http://bootstrap-datepicker.readthedocs.org/en/latest/index.html
- *
- * @param {String} options.timezone: Optional timezone to use instead of UTC.
- */
-function DateEditor(options) {
-  // A string that is always `UTC` in the DateEditor, eases DateTimeEditor inheritance.
-  this.timezone = options.timezone || 'UTC';
-
-  this.dateFormat = options.field.widgetOptionsJson.peek().dateFormat;
-  this.locale = options.field.documentSettings.peek().locale;
-
-  // Update moment format string to represent a date unambiguously.
-  this.safeFormat = makeFullMomentFormat(this.dateFormat);
-
-  // Use the default local timezone to format the placeholder date.
-  const defaultTimezone = moment.tz.guess();
-  let placeholder = moment.tz(defaultTimezone).format(this.safeFormat);
-  if (options.readonly) {
-    // clear placeholder for readonly mode
-    placeholder = null;
-  }
-  TextEditor.call(this, _.defaults(options, { placeholder: placeholder }));
-
-  const cellValue = this.formatValue(options.cellValue, this.safeFormat, true);
-
-  // Set the edited value, if not explicitly given, to the formatted version of cellValue.
-  this.textInput.value = gutil.undef(options.state, options.editValue, cellValue);
-
-  if (!options.readonly) {
-    // Indicates whether keyboard navigation is active for the datepicker.
-    this._keyboardNav = false;
-
-    // Attach the datepicker.
-    this._datePickerWidget = $(this.textInput).datepicker({
-      keyboardNavigation: false,
-      forceParse: false,
-      todayHighlight: true,
-      todayBtn: 'linked',
-      assumeNearbyYear: TWO_DIGIT_YEAR_THRESHOLD,
-      // Datepicker supports most of the languages. They just need to be included in the bundle
-      // or by script tag, i.e.
-      // <script src="bootstrap-datepicker/dist/locales/bootstrap-datepicker.pl.min.js"></script>
-      language : this.getLanguage(),
-      // Use the stripped format converted to one suitable for the datepicker.
-      format: {
-        toDisplay: (date, format, language) => moment.utc(date).format(this.safeFormat),
-        toValue: (date, format, language) => {
-          const timestampSec = parseDate(date, {
-            dateFormat: this.safeFormat,
-            // datepicker reads date in utc (ie: using date.getUTCDate()).
-            timezone: 'UTC',
-          });
-          return (timestampSec === null) ? null : new Date(timestampSec * 1000);
-        },
-      },
-    });
-    this.autoDisposeCallback(() => this._datePickerWidget.datepicker('destroy'));
-
-    // NOTE: Datepicker interferes with normal enter and escape functionality. Add an event handler
-    // to the DatePicker to prevent interference with normal behavior.
-    this._datePickerWidget.on('keydown', e => {
-      // If enter or escape is pressed, destroy the datepicker and re-dispatch the event.
-      if (e.keyCode === 13 || e.keyCode === 27) {
-        this._datePickerWidget.datepicker('destroy');
-        // The current target of the event will be the textarea.
-        setTimeout(() => e.currentTarget.dispatchEvent(e.originalEvent), 0);
-      }
-    });
-
-    // When the up/down arrow is pressed, modify the datepicker options to take control of
-    // the arrow keys for date selection.
-    let datepickerCommands = Object.assign({}, options.commands, {
-      datepickerFocus: () => { this._allowKeyboardNav(true); }
-    });
-    this._datepickerCommands = this.autoDispose(commands.createGroup(datepickerCommands, this, true));
-
-    this._datePickerWidget.on('show', () => {
-      // A workaround to allow clicking in the datepicker without losing focus.
-      dom(document.querySelector('.datepicker'),
-        kd.attr('tabIndex', 0),                   // allows datepicker to gain focus
-        kd.toggleClass('clipboard_focus', true)   // tells clipboard to not steal focus from us
-      );
-      // Attach command group to the input to allow switching keyboard focus to the datepicker.
-      dom(this.textInput,
-        // If the user inputs text into the textbox, take keyboard focus from the datepicker.
-        dom.on('input', () => { this._allowKeyboardNav(false); }),
-        this._datepickerCommands.attach()
-      );
-    });
-  }
-}
-
-dispose.makeDisposable(DateEditor);
-_.extend(DateEditor.prototype, TextEditor.prototype);
-
-/** @inheritdoc */
-DateEditor.prototype.getCellValue = function() {
-  let timestamp = parseDate(this.textInput.value, {
-    dateFormat: this.safeFormat,
-    timezone: this.timezone
-  });
-  return timestamp !== null ? timestamp : this.textInput.value;
-};
-
-// Helper to allow/disallow keyboard navigation within the datepicker.
-DateEditor.prototype._allowKeyboardNav = function(bool) {
-  if (this._keyboardNav !== bool) {
-    this._keyboardNav = bool;
-    $(this.textInput).data().datepicker.o.keyboardNavigation = bool;
-    // Force parse must be turned on with keyboard navigation, since it forces the highlighted date
-    // to be used when enter is pressed. Otherwise, keyboard date selection will have no effect.
-    $(this.textInput).data().datepicker.o.forceParse = bool;
-  }
-};
-
-// Moment value formatting helper.
-DateEditor.prototype.formatValue = function(value, formatString, shouldFallBackToValue) {
-  if (_.isNumber(value) && formatString) {
-    return moment.tz(value*1000, this.timezone).format(formatString);
-  } else {
-    // If value is AltText, return it unchanged. This way we can see it and edit in the editor.
-    return (shouldFallBackToValue && typeof value === 'string') ? value : "";
-  }
-};
-
-// Gets the language based on the current locale.
-DateEditor.prototype.getLanguage = function() {
-  // this requires a polyfill, i.e. https://www.npmjs.com/package/@formatjs/intl-locale
-  // more info about ts: https://github.com/microsoft/TypeScript/issues/37326
-  // return new Intl.Locale(locale).language;
-  return this.locale.substr(0, this.locale.indexOf("-"));
-}
-
-// Updates the given Moment format to specify a complete date, so that the datepicker sees an
-// unambiguous date in the textbox input. If the format is incomplete, fall back to YYYY-MM-DD.
-function makeFullMomentFormat(mFormat) {
-  let safeFormat = mFormat;
-  if (!safeFormat.includes('Y')) {
-    safeFormat += " YYYY";
-  }
-  if (!safeFormat.includes('D') || !safeFormat.includes('M')) {
-    safeFormat = 'YYYY-MM-DD';
-  }
-  return safeFormat;
-}
-
-module.exports = DateEditor;
diff --git a/app/client/widgets/DateEditor.ts b/app/client/widgets/DateEditor.ts
new file mode 100644
index 00000000..7da17108
--- /dev/null
+++ b/app/client/widgets/DateEditor.ts
@@ -0,0 +1,216 @@
+import {CommandGroup, createGroup} from 'app/client/components/commands';
+import {loadScript} from 'app/client/lib/loadScript';
+import {detectCurrentLang} from 'app/client/lib/localization';
+import {FieldOptions} from 'app/client/widgets/NewBaseEditor';
+import {NTextEditor} from 'app/client/widgets/NTextEditor';
+import {CellValue} from "app/common/DocActions";
+import {parseDate, TWO_DIGIT_YEAR_THRESHOLD} from 'app/common/parseDate';
+
+import moment from 'moment-timezone';
+import {dom} from 'grainjs';
+
+// These are all the locales available for the datepicker. Having a prepared list lets us find a
+// suitable one without trying combinations that don't exist. This list can be rebuilt using:
+//    ls bower_components/bootstrap-datepicker/dist/locales/bootstrap-datepicker.* | cut -d. -f2 | xargs echo
+// eslint-disable-next-line max-len
+const availableLocales = 'ar-tn ar az bg bm bn br bs ca cs cy da de el en-AU en-CA en-GB en-IE en-NZ en-ZA eo es et eu fa fi fo fr-CH fr gl he hi hr hu hy id is it-CH it ja ka kh kk km ko kr lt lv me mk mn ms nl-BE nl no oc pl pt-BR pt ro rs-latin rs ru si sk sl sq sr-latin sr sv sw ta tg th tk tr uk uz-cyrl uz-latn vi zh-CN zh-TW';
+
+monkeyPatchDatepicker();
+
+/**
+ * DateEditor - Editor for Date type. Includes a dropdown datepicker.
+ *  See reference: http://bootstrap-datepicker.readthedocs.org/en/latest/index.html
+ */
+export class DateEditor extends NTextEditor {
+  protected safeFormat: string;     // Format that specifies a complete date.
+
+  private _dateFormat: string|undefined = this.options.field.widgetOptionsJson.peek().dateFormat;
+  private _locale = detectCurrentLang();
+  private _keyboardNav = false;     // Whether keyboard navigation is active for the datepicker.
+
+  constructor(
+    options: FieldOptions,
+    protected timezone: string = 'UTC',     // For use by the derived DateTimeEditor.
+  ) {
+    super(options);
+
+    // Update moment format string to represent a date unambiguously.
+    this.safeFormat = makeFullMomentFormat(this._dateFormat || '');
+
+    // Set placeholder to current date(time), unless in read-only mode.
+    if (!options.readonly) {
+      // Use the default local timezone to format the placeholder date.
+      // TODO: this.timezone is better for DateTime; gristDoc.docInfo.timezone.peek() is better for Date.
+      const defaultTimezone = moment.tz.guess();
+      const placeholder = moment.tz(defaultTimezone).format(this.safeFormat);
+      this.textInput.setAttribute('placeholder', placeholder);
+    }
+
+    const cellValue = this.formatValue(options.cellValue, this.safeFormat, true);
+
+    // Set the edited value, if not explicitly given, to the formatted version of cellValue.
+    this.textInput.value = options.state ?? options.editValue ?? cellValue;
+
+    if (!options.readonly) {
+      // When the up/down arrow is pressed, modify the datepicker options to take control of
+      // the arrow keys for date selection.
+      const datepickerCommands = {
+        ...options.commands,
+        datepickerFocus: () => { this._allowKeyboardNav(true); }
+      };
+      const datepickerCommandGroup = this.autoDispose(createGroup(datepickerCommands, this, true));
+      this._attachDatePicker(datepickerCommandGroup)
+        .catch(e => console.error("Error attaching datepicker", e));
+    }
+  }
+
+  public getCellValue() {
+    const timestamp = parseDate(this.textInput.value, {
+      dateFormat: this.safeFormat,
+      timezone: this.timezone
+    });
+    return timestamp !== null ? timestamp : this.textInput.value;
+  }
+
+  // Moment value formatting helper.
+  protected formatValue(value: CellValue, formatString: string|undefined, shouldFallBackToValue: boolean) {
+    if (typeof value === 'number' && formatString) {
+      return moment.tz(value*1000, this.timezone).format(formatString);
+    } else {
+      // If value is AltText, return it unchanged. This way we can see it and edit in the editor.
+      return (shouldFallBackToValue && typeof value === 'string') ? value : "";
+    }
+  }
+
+  // Helper to allow/disallow keyboard navigation within the datepicker.
+  private _allowKeyboardNav(bool: boolean) {
+    if (this._keyboardNav !== bool) {
+      this._keyboardNav = bool;
+      $(this.textInput).data().datepicker.o.keyboardNavigation = bool;
+      // Force parse must be turned on with keyboard navigation, since it forces the highlighted date
+      // to be used when enter is pressed. Otherwise, keyboard date selection will have no effect.
+      $(this.textInput).data().datepicker.o.forceParse = bool;
+    }
+  }
+
+  // Attach the datepicker.
+  private async _attachDatePicker(datepickerCommands: CommandGroup) {
+    const localeToUse = await loadLocale(this._locale);
+    if (this.isDisposed()) { return; }    // Good idea to check after 'await'.
+    const datePickerWidget = $(this.textInput).datepicker({
+      keyboardNavigation: false,
+      forceParse: false,
+      todayHighlight: true,
+      todayBtn: 'linked',
+      assumeNearbyYear: TWO_DIGIT_YEAR_THRESHOLD,
+      language: localeToUse,
+      // Use the stripped format converted to one suitable for the datepicker.
+      format: {
+        toDisplay: (date: string, format: unknown, lang: unknown) => moment.utc(date).format(this.safeFormat),
+        toValue: (date: string, format: unknown, lang: unknown) => {
+          const timestampSec = parseDate(date, {
+            dateFormat: this.safeFormat,
+            // datepicker reads date in utc (ie: using date.getUTCDate()).
+            timezone: 'UTC',
+          });
+          return (timestampSec === null) ? null : new Date(timestampSec * 1000);
+        },
+      },
+    });
+    this.onDispose(() => datePickerWidget.datepicker('destroy'));
+
+    // NOTE: Datepicker interferes with normal enter and escape functionality. Add an event handler
+    // to the DatePicker to prevent interference with normal behavior.
+    datePickerWidget.on('keydown', (e) => {
+      // If enter or escape is pressed, destroy the datepicker and re-dispatch the event.
+      if (e.keyCode === 13 || e.keyCode === 27) {
+        datePickerWidget.datepicker('destroy');
+        // The current target of the event will be the textarea.
+        setTimeout(() => e.currentTarget?.dispatchEvent(e.originalEvent!), 0);
+      }
+    });
+
+    datePickerWidget.on('show', () => {
+      // A workaround to allow clicking in the datepicker without losing focus.
+      const datepickerElem: HTMLElement|null = document.querySelector('.datepicker');
+      if (datepickerElem) {
+        dom.update(datepickerElem,
+          dom.attr('tabIndex', '0'),      // allows datepicker to gain focus
+          dom.cls('clipboard_focus')      // tells clipboard to not steal focus from us
+        );
+      }
+
+      // Attach command group to the input to allow switching keyboard focus to the datepicker.
+      dom.update(this.textInput,
+        // If the user inputs text into the textbox, take keyboard focus from the datepicker.
+        dom.on('input', () => { this._allowKeyboardNav(false); }),
+        datepickerCommands.attach()
+      );
+    });
+    datePickerWidget.datepicker('show');
+  }
+}
+
+// Updates the given Moment format to specify a complete date, so that the datepicker sees an
+// unambiguous date in the textbox input. If the format is incomplete, fall back to YYYY-MM-DD.
+function makeFullMomentFormat(mFormat: string): string {
+  let safeFormat = mFormat;
+  if (!safeFormat.includes('Y')) {
+    safeFormat += " YYYY";
+  }
+  if (!safeFormat.includes('D') || !safeFormat.includes('M')) {
+    safeFormat = 'YYYY-MM-DD';
+  }
+  return safeFormat;
+}
+
+
+let availableLocaleSet: Set<string>|undefined;
+const loadedLocaleMap = new Map<string, string>();    // Maps requested locale to the one to use.
+
+// Datepicker supports many languages. They just need to be loaded. Here we load the language we
+// need on-demand, taking care not to load any language more than once (we don't need to assume
+// there is only one language being used on the page, though in practice that may well be true).
+async function loadLocale(locale: string): Promise<string> {
+  return loadedLocaleMap.get(locale) ||
+    loadedLocaleMap.set(locale, await doLoadLocale(locale)).get(locale)!;
+}
+
+async function doLoadLocale(locale: string): Promise<string> {
+  if (!availableLocaleSet) {
+    availableLocaleSet = new Set(availableLocales.split(/\s+/));
+  }
+  if (!availableLocaleSet.has(locale)) {
+    const shortLocale = locale.split("-")[0];            // If "xx-YY" is not available, try "xx"
+    if (!availableLocaleSet.has(shortLocale)) {
+      // No special locale available. (This is even true for "en", which is fine since that's
+      // loaded by default.)
+      return locale;
+    }
+    locale = shortLocale;
+  }
+
+  console.debug(`DateEditor: loading locale ${locale}`);
+  try {
+    await loadScript(`bootstrap-datepicker/dist/locales/bootstrap-datepicker.${locale}.min.js`);
+  } catch (e) {
+    console.warn(`DateEditor: failed to load ${locale}`);
+  }
+  return locale;
+}
+
+// DatePicker unfortunately requires an <input> (not <textarea>). But textarea is better for us,
+// because sometimes it's taller than a line, and an <input> looks worse. The following
+// unconsionable hack tricks Datepicker into thinking anything it's attached to is an input.
+// It's more reasonable to just modify boostrap-datepicker, but that has its own downside (with
+// upgrading and minification). This hack, however, is simpler than other workarounds.
+function monkeyPatchDatepicker() {
+  const Datepicker = ($.fn as any).datepicker?.Constructor;
+  if (Datepicker?.prototype) {
+    // datepicker.isInput can now be set to anything, but when read, always returns true. Tricksy.
+    Object.defineProperty(Datepicker.prototype, 'isInput', {
+      get: function() { return true; },
+      set: function(v) {},
+    });
+  }
+}
diff --git a/app/client/widgets/DateTimeEditor.js b/app/client/widgets/DateTimeEditor.js
deleted file mode 100644
index a4ba1edc..00000000
--- a/app/client/widgets/DateTimeEditor.js
+++ /dev/null
@@ -1,173 +0,0 @@
-/* global document */
-const moment = require('moment-timezone');
-const _ = require('underscore');
-const dom = require('../lib/dom');
-const dispose = require('../lib/dispose');
-const kd = require('../lib/koDom');
-const DateEditor = require('./DateEditor');
-const gutil = require('app/common/gutil');
-const { parseDate } = require('app/common/parseDate');
-const TextEditor = require('./TextEditor');
-
-/**
- * DateTimeEditor - Editor for DateTime type. Includes a dropdown datepicker.
- *  See reference: http://bootstrap-datepicker.readthedocs.org/en/latest/index.html
- */
-function DateTimeEditor(options) {
-  // Get the timezone from the end of the type string.
-  options.timezone = gutil.removePrefix(options.field.column().type(), "DateTime:");
-
-  // Adjust the command group.
-  var origCommands = options.commands;
-
-  // don't modify navigation for readonly mode
-  if (!options.readonly) {
-    options.commands = Object.assign({}, origCommands, {
-      prevField: () => this._focusIndex() === 1 ? this._setFocus(0) : origCommands.prevField(),
-      nextField: () => this._focusIndex() === 0 ? this._setFocus(1) : origCommands.nextField(),
-    });
-  }
-
-  // Call the superclass.
-  DateEditor.call(this, options);
-
-  this._timeFormat = options.field.widgetOptionsJson.peek().timeFormat;
-
-  // To reuse code, this knows all about the DOM that DateEditor builds (using TextEditor), and
-  // modifies that to be two side-by-side textareas.
-  this._dateSizer = this.contentSizer;    // For consistency with _timeSizer and _timeInput.
-  this._dateInput = this.textInput;
-
-  const isValid = _.isNumber(options.cellValue);
-  const formatted = this.formatValue(options.cellValue, this._timeFormat, false);
-  // Use a placeholder of 12:00am, since that is the autofill time value.
-  const placeholder = moment.tz('0', 'H', this.timezone).format(this._timeFormat);
-
-  // for readonly
-  if (options.readonly) {
-    if (!isValid) {
-      // do nothing - DateEditor will show correct error
-    } else {
-      // append time format or a placeholder
-      const time = (formatted || placeholder);
-      const sep = time ? ' ' : '';
-      this.textInput.value = this.textInput.value + sep + time;
-    }
-  } else {
-    dom(this.dom, kd.toggleClass('celleditor_datetime', true));
-    dom(this.dom.firstChild, kd.toggleClass('celleditor_datetime_editor', true));
-    this.dom.appendChild(
-      dom('div.celleditor_cursor_editor.celleditor_datetime_editor',
-        this._timeSizer = dom('div.celleditor_content_measure'),
-        this._timeInput = dom('textarea.celleditor_text_editor',
-          kd.attr('placeholder', placeholder),
-          kd.value(formatted),
-          this.commandGroup.attach(),
-          dom.on('input', () => this.onChange())
-        )
-      )
-    );
-  }
-
-  // If the edit value is encoded json, use those values as a starting point
-  if (typeof options.state == 'string') {
-    try {
-      const { date, time } = JSON.parse(options.state);
-      this._dateInput.value = date;
-      this._timeInput.value = time;
-      this.onChange();
-    } catch(e) {
-      console.error("DateTimeEditor can't restore its previous state")
-    }
-  }
-}
-
-dispose.makeDisposable(DateTimeEditor);
-_.extend(DateTimeEditor.prototype, DateEditor.prototype);
-
-DateTimeEditor.prototype.setSizerLimits = function() {
-  var maxSize = this.editorPlacement.calcSize({width: Infinity, height: Infinity}, {calcOnly: true});
-  if (this.options.readonly) {
-    return;
-  }
-  this._dateSizer.style.maxWidth =
-    this._timeSizer.style.maxWidth = Math.ceil(maxSize.width / 2 - 6) + 'px';
-};
-
-/**
- * Returns which element has focus: 0 if date, 1 if time, null if neither.
- */
-DateTimeEditor.prototype._focusIndex = function() {
-  return document.activeElement === this._dateInput ? 0 :
-    (document.activeElement === this._timeInput ? 1 : null);
-};
-
-/**
- * Sets focus to date if index is 0, or time if index is 1.
- */
-DateTimeEditor.prototype._setFocus = function(index) {
-  var elem = (index === 0 ? this._dateInput : (index === 1 ? this._timeInput : null));
-  if (elem) {
-    elem.focus();
-    elem.selectionStart = 0;
-    elem.selectionEnd = elem.value.length;
-  }
-};
-
-/**
- * Occurs when user types something into the editor
- */
-DateTimeEditor.prototype.onChange = function() {
-  this._resizeInput();
-
-  // store editor state as an encoded JSON string
-  const date = this._dateInput.value;
-  const time = this._timeInput.value;
-  this.editorState.set(JSON.stringify({ date, time}));
-}
-
-DateTimeEditor.prototype.getCellValue = function() {
-  let date = this._dateInput.value;
-  let time = this._timeInput.value;
-  let timestamp = parseDate(date, {
-    dateFormat: this.safeFormat,
-    time: time,
-    timeFormat: this._timeFormat,
-    timezone: this.timezone
-  });
-  return timestamp !== null ? timestamp :
-    (date && time ? `${date} ${time}` : date || time);
-};
-
-/**
- * Overrides the resizing function in TextEditor.
- */
-DateTimeEditor.prototype._resizeInput = function() {
-
-  // for readonly field, we will use logic from a super class
-  if (this.options.readonly) {
-    TextEditor.prototype._resizeInput.call(this);
-    return;
-  }
-  // Use the size calculation provided in options.calcSize (that takes into account cell size and
-  // screen size), with both date and time parts as the input. The resulting size is applied to
-  // the parent (containing date + time), with date and time each expanding or shrinking from the
-  // measured sizes using flexbox logic.
-  this._dateSizer.textContent = this._dateInput.value;
-  this._timeSizer.textContent = this._timeInput.value;
-  var dateRect = this._dateSizer.getBoundingClientRect();
-  var timeRect = this._timeSizer.getBoundingClientRect();
-  // Textboxes get 3px of padding on left/right/top (see TextEditor.css); we specify it manually
-  // since editorPlacement can't do a good job figuring it out with the flexbox arrangement.
-  var size = this.editorPlacement.calcSize({
-    width: dateRect.width + timeRect.width + 12,
-    height: Math.max(dateRect.height, timeRect.height) + 3
-  });
-  this.dom.style.width = size.width + 'px';
-  this._dateInput.parentNode.style.flexBasis = (dateRect.width + 6) + 'px';
-  this._timeInput.parentNode.style.flexBasis = (timeRect.width + 6) + 'px';
-  this._dateInput.style.height = Math.ceil(size.height - 3) + 'px';
-  this._timeInput.style.height = Math.ceil(size.height - 3) + 'px';
-};
-
-module.exports = DateTimeEditor;
diff --git a/app/client/widgets/DateTimeEditor.ts b/app/client/widgets/DateTimeEditor.ts
new file mode 100644
index 00000000..5e40b2f8
--- /dev/null
+++ b/app/client/widgets/DateTimeEditor.ts
@@ -0,0 +1,173 @@
+import {DateEditor} from 'app/client/widgets/DateEditor';
+import {FieldOptions} from 'app/client/widgets/NewBaseEditor';
+import {removePrefix} from 'app/common/gutil';
+import {parseDate} from 'app/common/parseDate';
+
+import moment from 'moment-timezone';
+import {dom} from 'grainjs';
+
+/**
+ * DateTimeEditor - Editor for DateTime type. Includes a dropdown datepicker.
+ *  See reference: http://bootstrap-datepicker.readthedocs.org/en/latest/index.html
+ */
+export class DateTimeEditor extends DateEditor {
+  private _timeFormat: string|undefined;
+  private _dateSizer: HTMLElement;
+  private _timeSizer: HTMLElement;
+  private _dateInput: HTMLTextAreaElement;
+  private _timeInput: HTMLTextAreaElement;
+
+  constructor(options: FieldOptions) {
+    // Get the timezone from the end of the type string.
+    const timezone = removePrefix(options.field.column().type(), "DateTime:");
+
+    // Adjust the command group, but not for readonly mode.
+    if (!options.readonly) {
+      const origCommands = options.commands;
+      options.commands = {
+        ...origCommands,
+        prevField: () => this._focusIndex() === 1 ? this._setFocus(0) : origCommands.prevField(),
+        nextField: () => this._focusIndex() === 0 ? this._setFocus(1) : origCommands.nextField(),
+      };
+    }
+
+    // Call the superclass.
+    super(options, timezone || 'UTC');
+    this._timeFormat = this.options.field.widgetOptionsJson.peek().timeFormat;
+
+    // To reuse code, this knows all about the DOM that DateEditor builds (using TextEditor), and
+    // modifies that to be two side-by-side textareas.
+    this._dateSizer = this.contentSizer;    // For consistency with _timeSizer.
+    this._dateInput = this.textInput;       // For consistency with _timeInput.
+
+    const isValid = (typeof options.cellValue === 'number');
+    const formatted = this.formatValue(options.cellValue, this._timeFormat, false);
+    // Use a placeholder of 12:00am, since that is the autofill time value.
+    const placeholder = moment.tz('0', 'H', this.timezone).format(this._timeFormat);
+
+    // for readonly
+    if (options.readonly) {
+      if (!isValid) {
+        // do nothing - DateEditor will show correct error
+      } else {
+        // append time format or a placeholder
+        const time = (formatted || placeholder);
+        const sep = time ? ' ' : '';
+        this.textInput.value = this.textInput.value + sep + time;
+      }
+    } else {
+      const widgetElem = this.getDom();
+      dom.update(widgetElem, dom.cls('celleditor_datetime'));
+      dom.update(this.cellEditorDiv, dom.cls('celleditor_datetime_editor'));
+      widgetElem.appendChild(
+        dom('div',
+          dom.cls('celleditor_cursor_editor'),
+          dom.cls('celleditor_datetime_editor'),
+          this._timeSizer = dom('div', dom.cls('celleditor_content_measure')),
+          this._timeInput = dom('textarea', dom.cls('celleditor_text_editor'),
+            dom.attr('placeholder', placeholder),
+            dom.prop('value', formatted),
+            this.commandGroup.attach(),
+            dom.on('input', () => this._onChange())
+          )
+        )
+      );
+    }
+
+    // If the edit value is encoded json, use those values as a starting point
+    if (typeof options.state == 'string') {
+      try {
+        const { date, time } = JSON.parse(options.state);
+        this._dateInput.value = date;
+        this._timeInput.value = time;
+        this._onChange();
+      } catch(e) {
+        console.error("DateTimeEditor can't restore its previous state");
+      }
+    }
+  }
+
+  public getCellValue() {
+    const date = this._dateInput.value;
+    const time = this._timeInput.value;
+    const timestamp = parseDate(date, {
+      dateFormat: this.safeFormat,
+      time: time,
+      timeFormat: this._timeFormat,
+      timezone: this.timezone
+    });
+    return timestamp !== null ? timestamp :
+      (date && time ? `${date} ${time}` : date || time);
+  }
+
+  public setSizerLimits() {
+    const maxSize = this.editorPlacement.calcSize({width: Infinity, height: Infinity}, {calcOnly: true});
+    if (this.options.readonly) {
+      return;
+    }
+    this._dateSizer.style.maxWidth =
+      this._timeSizer.style.maxWidth = Math.ceil(maxSize.width / 2 - 6) + 'px';
+  }
+
+  /**
+   * Overrides the resizing function in TextEditor.
+   */
+  protected resizeInput() {
+
+    // for readonly field, we will use logic from a super class
+    if (this.options.readonly) {
+      return super.resizeInput();
+    }
+    // Use the size calculation provided in options.calcSize (that takes into account cell size and
+    // screen size), with both date and time parts as the input. The resulting size is applied to
+    // the parent (containing date + time), with date and time each expanding or shrinking from the
+    // measured sizes using flexbox logic.
+    this._dateSizer.textContent = this._dateInput.value;
+    this._timeSizer.textContent = this._timeInput.value;
+    const dateRect = this._dateSizer.getBoundingClientRect();
+    const timeRect = this._timeSizer.getBoundingClientRect();
+    // Textboxes get 3px of padding on left/right/top (see TextEditor.css); we specify it manually
+    // since editorPlacement can't do a good job figuring it out with the flexbox arrangement.
+    const size = this.editorPlacement.calcSize({
+      width: dateRect.width + timeRect.width + 12,
+      height: Math.max(dateRect.height, timeRect.height) + 3
+    });
+    this.getDom().style.width = size.width + 'px';
+    this._dateInput.parentElement!.style.flexBasis = (dateRect.width + 6) + 'px';
+    this._timeInput.parentElement!.style.flexBasis = (timeRect.width + 6) + 'px';
+    this._dateInput.style.height = Math.ceil(size.height - 3) + 'px';
+    this._timeInput.style.height = Math.ceil(size.height - 3) + 'px';
+  }
+
+  /**
+   * Returns which element has focus: 0 if date, 1 if time, null if neither.
+   */
+  private _focusIndex() {
+    return document.activeElement === this._dateInput ? 0 :
+      (document.activeElement === this._timeInput ? 1 : null);
+  }
+
+  /**
+   * Sets focus to date if index is 0, or time if index is 1.
+   */
+  private _setFocus(index: 0|1) {
+    const elem = (index === 0 ? this._dateInput : (index === 1 ? this._timeInput : null));
+    if (elem) {
+      elem.focus();
+      elem.selectionStart = 0;
+      elem.selectionEnd = elem.value.length;
+    }
+  }
+
+  /**
+   * Occurs when user types something into the editor
+   */
+  private _onChange() {
+    this.resizeInput();
+
+    // store editor state as an encoded JSON string
+    const date = this._dateInput.value;
+    const time = this._timeInput.value;
+    this.editorState.set(JSON.stringify({ date, time}));
+  }
+}
diff --git a/app/client/widgets/FieldBuilder.ts b/app/client/widgets/FieldBuilder.ts
index 35d93221..fdf18dcf 100644
--- a/app/client/widgets/FieldBuilder.ts
+++ b/app/client/widgets/FieldBuilder.ts
@@ -29,7 +29,7 @@ import { FieldEditor, saveWithoutEditor } from 'app/client/widgets/FieldEditor';
 import { CellDiscussionPopup, EmptyCell } from 'app/client/widgets/DiscussionEditor';
 import { openFormulaEditor } from 'app/client/widgets/FormulaEditor';
 import { NewAbstractWidget } from 'app/client/widgets/NewAbstractWidget';
-import { NewBaseEditor } from "app/client/widgets/NewBaseEditor";
+import { IEditorConstructor } from "app/client/widgets/NewBaseEditor";
 import * as UserType from 'app/client/widgets/UserType';
 import * as UserTypeImpl from 'app/client/widgets/UserTypeImpl';
 import * as gristTypes from 'app/common/gristTypes';
@@ -758,7 +758,7 @@ export class FieldBuilder extends Disposable {
       return clearOwn();
     }
 
-    const editorCtor: typeof NewBaseEditor =
+    const editorCtor: IEditorConstructor =
       UserTypeImpl.getEditorConstructor(this.options().widget, this._readOnlyPureType());
     // constructor may be null for a read-only non-formula field, though not today.
     if (!editorCtor) {
diff --git a/app/client/widgets/FieldEditor.ts b/app/client/widgets/FieldEditor.ts
index 23634d91..87c4cf88 100644
--- a/app/client/widgets/FieldEditor.ts
+++ b/app/client/widgets/FieldEditor.ts
@@ -8,7 +8,7 @@ import {ViewFieldRec} from 'app/client/models/entities/ViewFieldRec';
 import {reportError} from 'app/client/models/errors';
 import {showTooltipToCreateFormula} from 'app/client/widgets/EditorTooltip';
 import {FormulaEditor, getFormulaError} from 'app/client/widgets/FormulaEditor';
-import {IEditorCommandGroup, NewBaseEditor} from 'app/client/widgets/NewBaseEditor';
+import {IEditorCommandGroup, IEditorConstructor, NewBaseEditor} from 'app/client/widgets/NewBaseEditor';
 import {asyncOnce} from "app/common/AsyncCreate";
 import {CellValue} from "app/common/DocActions";
 import * as gutil from 'app/common/gutil';
@@ -18,8 +18,6 @@ import {CursorPos} from 'app/plugin/GristAPI';
 import isEqual = require('lodash/isEqual');
 import {Disposable, dom, Emitter, Holder, MultiHolder, Observable} from 'grainjs';
 
-type IEditorConstructor = typeof NewBaseEditor;
-
 const t = makeT('FieldEditor');
 
 /**
diff --git a/app/client/widgets/FormulaEditor.ts b/app/client/widgets/FormulaEditor.ts
index 80ea836b..513b7ad3 100644
--- a/app/client/widgets/FormulaEditor.ts
+++ b/app/client/widgets/FormulaEditor.ts
@@ -118,11 +118,12 @@ export class FormulaEditor extends NewBaseEditor {
 
     const hideErrDetails = Observable.create(this, true);
     const raisedException = Computed.create(this, use => {
-      if (!options.formulaError || !use(options.formulaError)) {
+      const formulaError = options.formulaError && use(options.formulaError);
+      if (!formulaError) {
         return null;
       }
-      const error = isRaisedException(use(options.formulaError)!) ?
-                    decodeObject(use(options.formulaError)!) as RaisedException:
+      const error = isRaisedException(formulaError) ?
+                    decodeObject(formulaError) as RaisedException:
                     new RaisedException(["Unknown error"]);
       return error;
     });
@@ -382,7 +383,7 @@ export class FormulaEditor extends NewBaseEditor {
 
     // If we have an error to show, ask for a larger size for formulaEditor.
     const desiredSize = {
-      width: Math.max(desiredElemSize.width, (this.options.formulaError.get() ? minFormulaErrorWidth : 0)),
+      width: Math.max(desiredElemSize.width, (this.options.formulaError?.get() ? minFormulaErrorWidth : 0)),
       // Ask for extra space for the error; we'll decide how to allocate it below.
       height: desiredElemSize.height + (errorBoxDesiredHeight - errorBoxStartHeight),
     };
@@ -488,7 +489,7 @@ export function openFormulaEditor(options: {
   column?: ColumnRec,
   // Associated formula from a view field. If provided together with column, this field is used
   field?: ViewFieldRec,
-  editingFormula?: ko.Computed<boolean>,
+  editingFormula: ko.Computed<boolean>,
   // Needed to get exception value, if any.
   editRow?: DataRowModel,
   // Element over which to position the editor.
@@ -555,7 +556,7 @@ export function openFormulaEditor(options: {
     column,
     field: options.field,
   }) : undefined;
-  const editor = FormulaEditor.create(null, {
+  const editorOptions: IFormulaEditorOptions = {
     gristDoc,
     column,
     field: options.field,
@@ -569,7 +570,8 @@ export function openFormulaEditor(options: {
     cssClass: 'formula_editor_sidepane',
     readonly : false,
     canDetach: options.canDetach
-  } as IFormulaEditorOptions) as FormulaEditor;
+  };
+  const editor = FormulaEditor.create(null, editorOptions);
   editor.autoDispose(attachedHolder);
   editor.attach(refElem);
 
diff --git a/app/client/widgets/NTextEditor.ts b/app/client/widgets/NTextEditor.ts
index 8a6cb445..e6f7cf87 100644
--- a/app/client/widgets/NTextEditor.ts
+++ b/app/client/widgets/NTextEditor.ts
@@ -96,6 +96,14 @@ export class NTextEditor extends NewBaseEditor {
     this._contentSizer.style.maxWidth = Math.ceil(maxSize.width) + 'px';
   }
 
+  public get contentSizer(): HTMLElement {
+    return this._contentSizer;
+  }
+
+  public get editorPlacement(): EditorPlacement {
+    return this._editorPlacement;
+  }
+
   /**
    * Occurs when user types text in the textarea
    *
diff --git a/app/client/widgets/NewBaseEditor.ts b/app/client/widgets/NewBaseEditor.ts
index 69846a70..71edb8a8 100644
--- a/app/client/widgets/NewBaseEditor.ts
+++ b/app/client/widgets/NewBaseEditor.ts
@@ -13,11 +13,13 @@ export interface IEditorCommandGroup {
   [cmd: string]: () => void;
 }
 
+// Usually an editor is created for a field and provided FieldOptions, but it's possible to have
+// no field object, e.g. for a FormulaEditor for a conditional style rule.
 export interface Options {
   gristDoc: GristDoc;
   cellValue: CellValue;
   rowId: number;
-  formulaError: Observable<CellValue|undefined>;
+  formulaError?: Observable<CellValue|undefined>;
   editValue?: string;
   cursorPos: number;
   commands: IEditorCommandGroup;
@@ -29,6 +31,9 @@ export interface FieldOptions extends Options {
   field: ViewFieldRec;
 }
 
+// This represents any of the derived editor classes; the part after "&" restricts to non-abstract ones.
+export type IEditorConstructor = typeof NewBaseEditor & { new (...args: any[]): NewBaseEditor };
+
 /**
  * Required parameters:
  * @param {RowModel} options.field: ViewSectionField (i.e. column) being edited.
@@ -44,8 +49,10 @@ export abstract class NewBaseEditor extends Disposable {
    * Editors and provided by FieldBuilder. TODO: remove this method once all editors have been
    * updated to new-style Disposables.
    */
-  public static create<Opt extends Options>(owner: IDisposableOwner|null, options: Opt): NewBaseEditor;
-  public static create<Opt extends Options>(options: Opt): NewBaseEditor;
+  public static create<T extends new (...args: any[]) => any, Opt extends Options>(
+    this: T, owner: IDisposableOwner|null, options: Opt): InstanceType<T>;
+  public static create<T extends new (...args: any[]) => any, Opt extends Options>(
+    this: T, options: Opt): InstanceType<T>;
   public static create(ownerOrOptions: any, options?: any): NewBaseEditor {
     return options ?
       Disposable.create.call(this as any, ownerOrOptions, options) :
diff --git a/app/client/widgets/UserTypeImpl.ts b/app/client/widgets/UserTypeImpl.ts
index 89aece77..3b66c388 100644
--- a/app/client/widgets/UserTypeImpl.ts
+++ b/app/client/widgets/UserTypeImpl.ts
@@ -5,15 +5,15 @@ import ChoiceEditor from 'app/client/widgets/ChoiceEditor';
 import {ChoiceListCell} from 'app/client/widgets/ChoiceListCell';
 import {ChoiceListEditor} from 'app/client/widgets/ChoiceListEditor';
 import {ChoiceTextBox} from 'app/client/widgets/ChoiceTextBox';
-import DateEditor from 'app/client/widgets/DateEditor';
+import {DateEditor} from 'app/client/widgets/DateEditor';
 import DateTextBox from 'app/client/widgets/DateTextBox';
-import DateTimeEditor from 'app/client/widgets/DateTimeEditor';
+import {DateTimeEditor} from 'app/client/widgets/DateTimeEditor';
 import DateTimeTextBox from 'app/client/widgets/DateTimeTextBox';
 import {HyperLinkEditor} from 'app/client/widgets/HyperLinkEditor';
 import {HyperLinkTextBox} from 'app/client/widgets/HyperLinkTextBox';
 import {MarkdownTextBox} from 'app/client/widgets/MarkdownTextBox';
 import {NewAbstractWidget} from 'app/client/widgets/NewAbstractWidget';
-import {NewBaseEditor} from 'app/client/widgets/NewBaseEditor';
+import {IEditorConstructor} from 'app/client/widgets/NewBaseEditor';
 import {NTextBox} from 'app/client/widgets/NTextBox';
 import {NTextEditor} from 'app/client/widgets/NTextEditor';
 import {NumericEditor} from 'app/client/widgets/NumericEditor';
@@ -74,7 +74,7 @@ export function getFormWidgetConstructor(widget: string, type: string): WidgetCo
 }
 
 /** return a good class to instantiate for editing a widget/type combination */
-export function getEditorConstructor(widget: string, type: string): typeof NewBaseEditor {
+export function getEditorConstructor(widget: string, type: string): IEditorConstructor {
   const {config} = getWidgetConfiguration(widget, type as GristType);
   return nameToWidget[config.editCons as keyof typeof nameToWidget] as any;
 }
diff --git a/test/nbrowser/DateEditor.ts b/test/nbrowser/DateEditor.ts
new file mode 100644
index 00000000..6b467033
--- /dev/null
+++ b/test/nbrowser/DateEditor.ts
@@ -0,0 +1,166 @@
+import {assert, driver, Key, WebElement} from 'mocha-webdriver';
+import * as gu from 'test/nbrowser/gristUtils';
+import {setupTestSuite} from 'test/nbrowser/testUtils';
+import escapeRegExp = require('lodash/escapeRegExp');
+
+async function setCustomDateFormat(format: string) {
+  await gu.setDateFormat("Custom");
+  await driver.find('[data-test-id=Widget_dateCustomFormat]').click();
+  await gu.selectAll();
+  await driver.sendKeys(format, Key.ENTER);
+}
+
+async function testDateFormat(initialDateStr: string, newDay: string, finalDateStr: string) {
+  const cell = await gu.getCell({col: 'A', rowNum: 1});
+  await cell.click();
+  assert.equal(await cell.getText(), initialDateStr);
+
+  // Open the date for editing, and check that we see it in the new format.
+  await driver.sendKeys(Key.ENTER);
+  await gu.checkTextEditor(new RegExp(escapeRegExp(initialDateStr)));
+
+  // Pick a new date in the editor; check that it's shown in the new format.
+  await driver.findContent('td.day', newDay).click();
+  await gu.checkTextEditor(new RegExp(escapeRegExp(finalDateStr)));
+  await driver.sendKeys(Key.ENTER);
+  await gu.waitForServer();
+  assert.equal(await gu.getCell({col: 'A', rowNum: 1}).getText(), finalDateStr);
+
+  // Reopen the editor, check that our previously-selected date is still selected.
+  await gu.getCell({col: 'A', rowNum: 1}).click();
+  await driver.sendKeys(Key.ENTER);
+  await gu.checkTextEditor(new RegExp(escapeRegExp(finalDateStr)));
+  assert.isTrue(await driver.findContent('td.day', newDay).matches('.active'));
+  await driver.sendKeys(Key.ESCAPE);
+  await gu.waitAppFocus();
+  await gu.undo();
+}
+
+describe('DateEditor', function() {
+  this.timeout(20000);
+  const cleanup = setupTestSuite();
+  let session: gu.Session;
+
+  afterEach(() => gu.checkForErrors());
+
+  before(async function() {
+    session = await gu.session().login();
+    await session.tempNewDoc(cleanup, 'DateEditor');
+    await gu.waitForServer();
+    await driver.executeAsyncScript(async (done: () => unknown) => {
+      await (window as any).loadScript('sinon.js');
+      window.sinon.useFakeTimers({
+        now: 1580568300000,  // Sat Feb 01 2020 14:45:00 UTC
+        shouldAdvanceTime: true
+      });
+      done();
+    });
+  });
+
+  it('should allow editing dates in standard format', async function() {
+    await gu.getCell({col: 'A', rowNum: 1}).click();
+    await gu.setType(/Date/);
+    assert.equal(await gu.getDateFormat(), "YYYY-MM-DD");
+
+    // Use shortcut to populate today's date, mainly to ensure that our date-mocking is working.
+    await gu.getCell({col: 'A', rowNum: 1}).click();
+    await gu.sendKeys(Key.chord(await gu.modKey(), ';'));
+    await gu.waitForServer();
+    assert.equal(await gu.getCell({col: 'A', rowNum: 1}).getText(), '2020-02-01');
+
+    // Change the format and check that date gets updated.
+    await gu.setDateFormat("MMMM Do, YYYY");
+    await testDateFormat('February 1st, 2020', '18', 'February 18th, 2020');
+  });
+
+  it('should allow editing dates in rarer formats', async function() {
+    await setCustomDateFormat("MMM Do, 'YY");
+    await testDateFormat("Feb 1st, '20", "18", "Feb 18th, '20");
+
+    await setCustomDateFormat("YYYY-MM-DD dd");
+    await testDateFormat("2020-02-01 Sa", "18", "2020-02-18 Tu");
+  });
+
+  it('should allow editing invalid alt-text', async function() {
+    let cell = await gu.getCell({col: 'A', rowNum: 2});
+    await cell.click();
+    await driver.sendKeys(Key.ENTER);
+    await gu.waitAppFocus(false);
+
+    // Enter an invalid date.
+    await driver.sendKeys('2020-03-14pi', Key.ENTER);
+    await gu.waitForServer();
+
+    // Check that it's saved, and shows up as invalid.
+    cell = await gu.getCell({col: 'A', rowNum: 2});
+    assert.equal(await cell.getText(), '2020-03-14pi');
+    assert.isTrue(await cell.find('.field_clip').matches('.invalid'));
+
+    // Open for editing, and check that the invalid value is present in the editor.
+    await cell.click();
+    await driver.sendKeys(Key.ENTER);
+    await gu.waitAppFocus(false);
+    await gu.checkTextEditor(/2020-03-14pi/);
+
+    // Edit it down to something valid, save, and check.
+    await driver.sendKeys(Key.BACK_SPACE, Key.BACK_SPACE, Key.ENTER);
+    await gu.waitForServer();
+    cell = await gu.getCell({col: 'A', rowNum: 2});
+    assert.equal(await cell.getText(), '2020-03-14 Sa');
+    assert.isFalse(await cell.find('.field_clip').matches('.invalid'));
+  });
+
+  async function openCellEditor(cell: WebElement) {
+    await cell.click();
+    await driver.sendKeys(Key.ENTER);
+    await gu.waitAppFocus(false);
+  }
+
+  it('should respect locale for datepicker', async function() {
+    let cell = await gu.getCell({col: 'A', rowNum: 1});
+    await cell.click();
+    await gu.setDateFormat("YYYY-MM-DD");
+
+    assert.equal(await cell.getText(), '2020-02-01');
+    await openCellEditor(cell);
+
+    // Check that the date input contains the correct date.
+    assert.equal(await driver.find('.celleditor_text_editor').value(), '2020-02-01');
+
+    // Wait for datepicker, and check that it's showing the expected (default English) locale.
+    await driver.findWait('.datepicker', 200);
+    assert.equal(await driver.find('.datepicker .datepicker-days .datepicker-switch').getText(), 'February 2020');
+    assert.deepEqual(await driver.findAll('.datepicker .datepicker-days .dow', el => el.getText()),
+      ['Su', 'Mo', 'Tu', 'We', 'Th', 'Fr', 'Sa']);
+
+    // Check that it works to click into it to change date.
+    await driver.findContent('.datepicker .day', '19').click();
+    assert.equal(await driver.find('.celleditor_text_editor').value(), '2020-02-19');
+    await driver.sendKeys(Key.ENTER);
+    await gu.waitForServer();
+    assert.equal(await cell.getText(), '2020-02-19');
+    assert.equal(await driver.find('.datepicker').isPresent(), false);
+
+    // Change locale to something quite different.
+    const api = session.createHomeApi();
+    await api.updateUserLocale('fr-CA');
+    cleanup.addAfterEach(() => api.updateUserLocale(null));   // Restore after this test case.
+    await gu.reloadDoc();
+
+    // Check that the datepicker now opens to show the new language.
+    cell = await gu.getCell({col: 'A', rowNum: 1});
+    await openCellEditor(cell);
+    assert.equal(await driver.find('.datepicker .datepicker-days .datepicker-switch').getText(),
+      'février 2020');
+    assert.deepEqual(await driver.findAll('.datepicker .datepicker-days .dow', el => el.getText()),
+      ['l', 'ma', 'me', 'j', 'v', 's', 'd']);
+
+    // Check that it can still be used to pick a new date.
+    await driver.findContent('.datepicker .day', '26').click();
+    assert.equal(await driver.find('.celleditor_text_editor').value(), '2020-02-26');
+    await driver.sendKeys(Key.ENTER);
+    await gu.waitForServer();
+    assert.equal(await cell.getText(), '2020-02-26');
+    assert.equal(await driver.find('.datepicker').isPresent(), false);
+  });
+});