diff --git a/app/client/components/TypeConversion.ts b/app/client/components/TypeConversion.ts
index 11cda7da..b3307535 100644
--- a/app/client/components/TypeConversion.ts
+++ b/app/client/components/TypeConversion.ts
@@ -9,6 +9,7 @@ import {ColumnRec} from 'app/client/models/entities/ColumnRec';
 import * as gristTypes from 'app/common/gristTypes';
 import {isFullReferencingType} from 'app/common/gristTypes';
 import * as gutil from 'app/common/gutil';
+import {dateTimeWidgetOptions, guessDateFormat} from 'app/common/parseDate';
 import {TableData} from 'app/common/TableData';
 import {decodeObject} from 'app/plugin/objtypes';
 
@@ -92,8 +93,21 @@ export async function prepTransformColInfo(docModel: DocModel, origCol: ColumnRe
     formula: "CURRENT_CONVERSION(rec)",
   };
 
-  const prevOptions = origCol.widgetOptionsJson.peek() || {};
+  const visibleCol = origCol.visibleColModel();
+  // Column used to derive previous widget options and sample values for guessing
+  const sourceCol = visibleCol.getRowId() !== 0 ? visibleCol : origCol;
+  const prevOptions = sourceCol.widgetOptionsJson.peek() || {};
   switch (toType) {
+    case 'Date':
+    case 'DateTime': {
+      let {dateFormat} = prevOptions;
+      if (!dateFormat) {
+        const colValues = tableData.getColValues(sourceCol.colId()) || [];
+        dateFormat = guessDateFormat(colValues.map(String)) || "YYYY-MM-DD";
+      }
+      widgetOptions = dateTimeWidgetOptions(dateFormat);
+      break;
+    }
     case 'Choice': {
       if (Array.isArray(prevOptions.choices)) {
         // Use previous choices if they are set, e.g. if converting from ChoiceList
@@ -101,8 +115,7 @@ export async function prepTransformColInfo(docModel: DocModel, origCol: ColumnRe
       } else {
         // Set suggested choices. Limit to 100, since too many choices is more likely to cause
         // trouble than desired behavior. For many choices, recommend using a Ref to helper table.
-        const colId = isReferenceCol(origCol) ? origDisplayCol.colId() : origCol.colId();
-        const columnData = tableData.getDistinctValues(colId, 100);
+        const columnData = tableData.getDistinctValues(sourceCol.colId(), 100);
         if (columnData) {
           columnData.delete("");
           columnData.delete(null);
@@ -119,8 +132,7 @@ export async function prepTransformColInfo(docModel: DocModel, origCol: ColumnRe
         // Set suggested choices. This happens before the conversion to ChoiceList, so we do some
         // light guessing for likely choices to suggest.
         const choices = new Set<string>();
-        const colId = isReferenceCol(origCol) ? origDisplayCol.colId() : origCol.colId();
-        for (let value of tableData.getColValues(colId) || []) {
+        for (let value of tableData.getColValues(sourceCol.colId()) || []) {
           if (value === null) { continue; }
           value = String(decodeObject(value)).trim();
           const tags: unknown[] = (value.startsWith('[') && gutil.safeJsonParse(value, null)) || value.split(",");
diff --git a/app/client/components/TypeTransform.ts b/app/client/components/TypeTransform.ts
index 6655d8e3..3941897d 100644
--- a/app/client/components/TypeTransform.ts
+++ b/app/client/components/TypeTransform.ts
@@ -15,10 +15,8 @@ import {basicButton, primaryButton} from 'app/client/ui2018/buttons';
 import {testId} from 'app/client/ui2018/cssVars';
 import {FieldBuilder} from 'app/client/widgets/FieldBuilder';
 import {NewAbstractWidget} from 'app/client/widgets/NewAbstractWidget';
-import {ColValues, UserAction} from 'app/common/DocActions';
+import {UserAction} from 'app/common/DocActions';
 import {Computed, dom, fromKo, Observable} from 'grainjs';
-import isEmpty = require('lodash/isEmpty');
-import pickBy = require('lodash/pickBy');
 
 // To simplify diff (avoid rearranging methods to satisfy private/public order).
 /* eslint-disable @typescript-eslint/member-ordering */
@@ -150,14 +148,7 @@ export class TypeTransform extends ColumnTransform {
   public async setType(toType: string) {
     const docModel = this.gristDoc.docModel;
     const colInfo = await TypeConversion.prepTransformColInfo(docModel, this.origColumn, this.origDisplayCol, toType);
-    // Only update those values which changed, and only if needed.
     const tcol = this.transformColumn;
-    const changedInfo = pickBy(colInfo, (val, key) =>
-      (val !== tcol[key as keyof TypeConversion.ColInfo].peek()));
-    if (!isEmpty(changedInfo)) {
-      // Update the transform column, particularly the type.
-      // This will trigger the subscription in postAddTransformColumn and lead to calling convertValues.
-      await tcol.updateColValues(changedInfo as ColValues);
-    }
+    await tcol.updateColValues(colInfo as any);
   }
 }
diff --git a/app/client/widgets/DateTextBox.js b/app/client/widgets/DateTextBox.js
index 87bb985a..2a93bb63 100644
--- a/app/client/widgets/DateTextBox.js
+++ b/app/client/widgets/DateTextBox.js
@@ -12,6 +12,7 @@ const {cssRow, cssLabel} = require('app/client/ui/RightPanel');
 const {cssTextInput} = require("app/client/ui2018/editableLabel");
 const {styled, fromKo} = require('grainjs');
 const {select} = require('app/client/ui2018/menus');
+const {dateFormatOptions} = require('app/common/parseDate');
 
 /**
  * DateTextBox - The most basic widget for displaying simple date information.
@@ -23,18 +24,6 @@ function DateTextBox(field) {
   this.dateFormat = this.options.prop('dateFormat');
   this.isCustomDateFormat = this.options.prop('isCustomDateFormat');
 
-  this.dateFormatOptions = [
-    'YYYY-MM-DD',
-    'MM-DD-YYYY',
-    'MM/DD/YYYY',
-    'MM-DD-YY',
-    'MM/DD/YY',
-    'DD MMM YYYY',
-    'MMMM Do, YYYY',
-    'DD-MM-YYYY',
-    'Custom'
-  ];
-
   // Helper to set 'dateFormat' and 'isCustomDateFormat' from the set of default date format strings.
   this.standardDateFormat = this.autoDispose(ko.computed({
     owner: this,
@@ -58,7 +47,7 @@ DateTextBox.prototype.buildDateConfigDom = function() {
   var self = this;
   return dom('div',
     cssLabel("Date Format"),
-    cssRow(dom(select(fromKo(self.standardDateFormat), self.dateFormatOptions), dom.testId("Widget_dateFormat"))),
+    cssRow(dom(select(fromKo(self.standardDateFormat), [...dateFormatOptions, "Custom"]), dom.testId("Widget_dateFormat"))),
     kd.maybe(self.isCustomDateFormat, function() {
       return cssRow(dom(textbox(self.dateFormat), dom.testId("Widget_dateCustomFormat")));
     })
diff --git a/app/client/widgets/DateTimeTextBox.js b/app/client/widgets/DateTimeTextBox.js
index 5ab8ab03..b398b1a4 100644
--- a/app/client/widgets/DateTimeTextBox.js
+++ b/app/client/widgets/DateTimeTextBox.js
@@ -15,6 +15,7 @@ const {cssTextInput} = require("app/client/ui2018/editableLabel");
 const {dom: gdom, styled, fromKo} = require('grainjs');
 const {select} = require('app/client/ui2018/menus');
 const {buildTZAutocomplete} = require('app/client/widgets/TZAutocomplete');
+const {timeFormatOptions} = require("app/common/parseDate");
 
 
 /**
@@ -32,16 +33,6 @@ function DateTimeTextBox(field) {
   this.timeFormat = this.options.prop('timeFormat');
   this.isCustomTimeFormat = this.options.prop('isCustomTimeFormat');
 
-  this.timeFormatOptions = [
-    'h:mma',
-    'h:mma z',
-    'HH:mm',
-    'HH:mm z',
-    'HH:mm:ss',
-    'HH:mm:ss z',
-    'Custom'
-  ];
-
   // Helper to set 'timeFormat' and 'isCustomTimeFormat' from the set of default time format strings.
   this.standardTimeFormat = this.autoDispose(ko.computed({
     owner: this,
@@ -72,7 +63,7 @@ DateTimeTextBox.prototype.buildConfigDom = function(isTransformConfig) {
       ),
     self.buildDateConfigDom(),
     cssLabel("Time Format"),
-    cssRow(dom(select(fromKo(self.standardTimeFormat), self.timeFormatOptions), dom.testId("Widget_timeFormat"))),
+    cssRow(dom(select(fromKo(self.standardTimeFormat), [...timeFormatOptions, "Custom"]), dom.testId("Widget_timeFormat"))),
     kd.maybe(self.isCustomTimeFormat, function() {
       return cssRow(dom(textbox(self.timeFormat), dom.testId("Widget_timeCustomFormat")));
     }),
diff --git a/app/common/TableData.ts b/app/common/TableData.ts
index 360c6817..1f06baad 100644
--- a/app/common/TableData.ts
+++ b/app/common/TableData.ts
@@ -5,7 +5,7 @@ import {ActionDispatcher} from 'app/common/ActionDispatcher';
 import {BulkColValues, CellValue, ColInfo, ColInfoWithId, ColValues, DocAction,
         isSchemaAction, ReplaceTableData, RowRecord, TableDataAction} from 'app/common/DocActions';
 import {getDefaultForType} from 'app/common/gristTypes';
-import {arrayRemove, arraySplice} from 'app/common/gutil';
+import {arrayRemove, arraySplice, getDistinctValues} from 'app/common/gutil';
 import {SchemaTypes} from "app/common/schema";
 import {UIRowId} from 'app/common/UIRowId';
 import isEqual = require('lodash/isEqual');
@@ -231,12 +231,7 @@ export class TableData extends ActionDispatcher implements SkippableRows {
   public getDistinctValues(colId: string, count: number = Infinity): Set<CellValue>|undefined {
     const valColumn = this.getColValues(colId);
     if (!valColumn) { return undefined; }
-    const distinct: Set<CellValue> = new Set();
-    // Add values to the set until it reaches the desired size, or until there are no more values.
-    for (let i = 0; i < valColumn.length && distinct.size < count; i++) {
-      distinct.add(valColumn[i]);
-    }
-    return distinct;
+    return getDistinctValues(valColumn, count);
   }
 
   /**
diff --git a/app/common/declarations.d.ts b/app/common/declarations.d.ts
index 32cc30a6..190e628f 100644
--- a/app/common/declarations.d.ts
+++ b/app/common/declarations.d.ts
@@ -23,3 +23,5 @@ declare namespace Intl {
     constructor(locale: string);
   }
 }
+
+declare module '@gristlabs/moment-guess/dist/bundle.js';
diff --git a/app/common/gutil.ts b/app/common/gutil.ts
index 9cbdc6bb..3826ed63 100644
--- a/app/common/gutil.ts
+++ b/app/common/gutil.ts
@@ -915,3 +915,15 @@ export const unwrap: UseCB = (obs: ISubscribable) => {
   }
   return (obs as ko.Observable).peek();
 };
+
+/**
+ * Get a set of up to `count` distinct values of `values`.
+ */
+export function getDistinctValues<T>(values: readonly T[], count: number = Infinity): Set<T> {
+  const distinct = new Set<T>();
+  // Add values to the set until it reaches the desired size, or until there are no more values.
+  for (let i = 0; i < values.length && distinct.size < count; i++) {
+    distinct.add(values[i]);
+  }
+  return distinct;
+}
diff --git a/app/common/parseDate.ts b/app/common/parseDate.ts
index a2463d94..2f17e567 100644
--- a/app/common/parseDate.ts
+++ b/app/common/parseDate.ts
@@ -1,5 +1,8 @@
 import escapeRegExp = require('lodash/escapeRegExp');
 import memoize = require('lodash/memoize');
+import {getDistinctValues} from 'app/common/gutil';
+// Simply importing 'moment-guess' inconsistently imports bundle.js or bundle.esm.js depending on environment
+import * as guessFormat from '@gristlabs/moment-guess/dist/bundle.js';
 import * as moment from 'moment-timezone';
 
 // When using YY format, use a consistent interpretation in datepicker and in moment parsing: add
@@ -321,3 +324,70 @@ function standardizeTime(timeString: string): { remaining: string, time: string
   const hh = String(hours).padStart(2, '0');
   return {remaining: timeString.slice(0, match.index).trim(), time: `${hh}:${mm}:${ss}`};
 }
+
+export function guessDateFormat(values: string[], timezone: string = 'UTC'): string | null {
+  const sample = getDistinctValues(values, 100);
+  const formats: Record<string, number> = {};
+  for (const dateString of sample) {
+    let guessed: string | string[];
+    try {
+      guessed = guessFormat(dateString);
+    } catch {
+      continue;
+    }
+    if (typeof guessed === "string") {
+      guessed = [guessed];
+    }
+    for (const guess of guessed) {
+      formats[guess] = 0;
+    }
+  }
+  const formatKeys = Object.keys(formats);
+  if (!formatKeys.length || formatKeys.length > 10) {
+    return null;
+  }
+
+  for (const format of formatKeys) {
+    for (const dateString of values) {
+      const m = moment.tz(dateString, format, true, timezone);
+      if (m.isValid()) {
+        formats[format] += 1;
+      }
+    }
+  }
+
+  const maxCount = Math.max(...Object.values(formats));
+  return formatKeys.find(format => formats[format] === maxCount)!;
+}
+
+export const dateFormatOptions = [
+  'YYYY-MM-DD',
+  'MM-DD-YYYY',
+  'MM/DD/YYYY',
+  'MM-DD-YY',
+  'MM/DD/YY',
+  'DD MMM YYYY',
+  'MMMM Do, YYYY',
+  'DD-MM-YYYY',
+];
+
+export const timeFormatOptions = [
+  'h:mma',
+  'h:mma z',
+  'HH:mm',
+  'HH:mm z',
+  'HH:mm:ss',
+  'HH:mm:ss z',
+];
+
+export function dateTimeWidgetOptions(fullFormat: string) {
+  const index = fullFormat.match(/[hHkaAmsSzZT]|$/)!.index!;
+  const dateFormat = fullFormat.substr(0, index).trim();
+  const timeFormat = fullFormat.substr(index).trim() || timeFormatOptions[0];
+  return {
+    dateFormat,
+    timeFormat,
+    isCustomDateFormat: !dateFormatOptions.includes(dateFormat),
+    isCustomTimeFormat: !timeFormatOptions.includes(timeFormat),
+  };
+}
diff --git a/package.json b/package.json
index ef7ef5d9..c230937b 100644
--- a/package.json
+++ b/package.json
@@ -78,6 +78,7 @@
     "@googleapis/oauth2": "0.2.0",
     "@gristlabs/connect-sqlite3": "0.9.11-grist.1",
     "@gristlabs/express-session": "1.17.0",
+    "@gristlabs/moment-guess": "1.2.4-grist.1",
     "@gristlabs/pidusage": "2.0.17",
     "@gristlabs/sqlite3": "4.1.1-grist.1",
     "@popperjs/core": "2.3.3",
diff --git a/test/nbrowser/gristUtils.ts b/test/nbrowser/gristUtils.ts
index 8089e0e8..27651ccb 100644
--- a/test/nbrowser/gristUtils.ts
+++ b/test/nbrowser/gristUtils.ts
@@ -1789,7 +1789,11 @@ export async function openDocumentSettings() {
  * Returns date format for date and datetime editor
  */
 export async function getDateFormat(): Promise<string> {
-  return driver.find('[data-test-id=Widget_dateFormat] .test-select-row').getText();
+  const result = await driver.find('[data-test-id=Widget_dateFormat] .test-select-row').getText();
+  if (result === "Custom") {
+    return driver.find('[data-test-id=Widget_dateCustomFormat] input').value();
+  }
+  return result;
 }
 
 /**
diff --git a/yarn.lock b/yarn.lock
index bd0a3404..1eaa8116 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -62,6 +62,14 @@
     safe-buffer "5.2.0"
     uid-safe "~2.1.5"
 
+"@gristlabs/moment-guess@1.2.4-grist.1":
+  version "1.2.4-grist.1"
+  resolved "https://registry.yarnpkg.com/@gristlabs/moment-guess/-/moment-guess-1.2.4-grist.1.tgz#b15daba4b9d1e1d4e4fbafbddc0be8dddc21dde3"
+  integrity sha512-emosrHFak1JYEjZmZoeLO+sjTD48Amx2NH/BRCmiCwH50fcK1VvfwrWHjRu2oQ5p8+yuZxHRxsm0+2KANaNLdg==
+  dependencies:
+    arg "^4.1.3"
+    chalk "^4.1.0"
+
 "@gristlabs/pidusage@2.0.17":
   version "2.0.17"
   resolved "https://registry.yarnpkg.com/@gristlabs/pidusage/-/pidusage-2.0.17.tgz#829f77dc9fc711cd4713b774e4fbcc4a71d6cdc5"
@@ -780,6 +788,11 @@ are-we-there-yet@~1.1.2:
     delegates "^1.0.0"
     readable-stream "^2.0.6"
 
+arg@^4.1.3:
+  version "4.1.3"
+  resolved "https://registry.yarnpkg.com/arg/-/arg-4.1.3.tgz#269fc7ad5b8e42cb63c896d5666017261c144089"
+  integrity sha512-58S9QDqG0Xx27YwPSt9fJxivjYl432YCwfDMfZ+71RAqUrZef7LrKQZ3LHLOwCS4FLNBplP533Zx895SeOCHvA==
+
 argparse@^1.0.7:
   version "1.0.10"
   resolved "https://registry.yarnpkg.com/argparse/-/argparse-1.0.10.tgz#bcd6791ea5ae09725e17e5ad988134cd40b3d911"
@@ -1529,6 +1542,14 @@ chalk@^4.0.0:
     ansi-styles "^4.1.0"
     supports-color "^7.1.0"
 
+chalk@^4.1.0:
+  version "4.1.2"
+  resolved "https://registry.yarnpkg.com/chalk/-/chalk-4.1.2.tgz#aac4e2b7734a740867aeb16bf02aad556a1e7a01"
+  integrity sha512-oKnbhFyRIXpUuez8iBMmyEa4nbj4IOQyuhc/wy9kY7/WVPcwIO9VA668Pu8RkO7+0G76SLROeyw9CpQ061i4mA==
+  dependencies:
+    ansi-styles "^4.1.0"
+    supports-color "^7.1.0"
+
 check-error@^1.0.2:
   version "1.0.2"
   resolved "https://registry.yarnpkg.com/check-error/-/check-error-1.0.2.tgz#574d312edd88bb5dd8912e9286dd6c0aed4aac82"