(core) Converting big number (9 digits or more) to date directly

Summary:
Interpret huge numbers (>8 digits) as timestamps when converting numeric column to date.
Convert date/date time columns to timestamp when converted from numeric/int column.

Test Plan: Updated

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: dsagal, alexmojaki

Differential Revision: https://phab.getgrist.com/D4030

app/client/components/TypeConversion.ts

@@ -11,6 +11,7 @@ import {csvDecodeRow} from 'app/common/csvFormat';
 import * as gristTypes from 'app/common/gristTypes';
 import {isFullReferencingType} from 'app/common/gristTypes';
 import * as gutil from 'app/common/gutil';
+import {isNonNullish} from 'app/common/gutil';
 import NumberParse from 'app/common/NumberParse';
 import {dateTimeWidgetOptions, guessDateFormat, timeFormatOptions} from 'app/common/parseDate';
 import {TableData} from 'app/common/TableData';
@@ -134,7 +135,8 @@ export async function prepTransformColInfo(docModel: DocModel, origCol: ColumnRe
       if (!dateFormat) {
         // Guess date and time format if there aren't any already
         const colValues = tableData.getColValues(sourceCol.colId()) || [];
-        dateFormat = guessDateFormat(colValues.map(String));
+        const strValues = colValues.map(v => isNonNullish(v) ? String(v) : null);
+        dateFormat = guessDateFormat(strValues);
         widgetOptions = {...widgetOptions, ...(dateTimeWidgetOptions(dateFormat, true))};
       }
       if (toType === 'DateTime' && !widgetOptions.timeFormat) {