Handle ambiguous dates (with no offset) in DateTimePickerControl (#35077)

* Add tests for ambiguous and unambiguous UTC dates

* Assume UTC for ambiguous dates

* Changelog

* Remove console statements

packages/js/components/changelog/fix-date-time-picker-control-ambiguous-date

@@ -0,0 +1,4 @@
+Significance: patch
+Type: tweak
+
+Assume ambiguous dates passed into DateTimePickerControl are UTC.

packages/js/components/src/date-time-picker-control/date-time-picker-control.tsx

@@ -75,20 +75,28 @@ export const DateTimePickerControl: React.FC< DateTimePickerControlProps > = ( {
 		null
 	);
 
-	function parseMomentIso( dateString?: string | null ): Moment {
+	function parseMomentIso(
-		return moment( dateString, moment.ISO_8601, true );
+		dateString?: string | null,
+		assumeLocalTime = false
+	): Moment {
+		if ( assumeLocalTime ) {
+			return moment( dateString, moment.ISO_8601, true ).utc();
+		}
+
+		return moment.utc( dateString, moment.ISO_8601, true );
 	}
 
 	function parseMoment( dateString?: string | null ): Moment {
+		// parse input date string as local time
 		return moment( dateString, dateTimeFormat );
 	}
 
 	function formatMomentIso( momentDate: Moment ): string {
-		return momentDate.toISOString();
+		return momentDate.utc().toISOString();
 	}
 
 	function formatMoment( momentDate: Moment ): string {
-		return momentDate.format( dateTimeFormat );
+		return momentDate.local().format( dateTimeFormat );
 	}
 
 	function hasFocusLeftInputAndDropdownContent(
@@ -272,8 +280,9 @@ export const DateTimePickerControl: React.FC< DateTimePickerControlProps > = ( {
 							: undefined
 					}
 					onChange={ ( date: string ) => {
+						// the picker returns the date in local time
 						const formattedDate = formatMoment(
-							parseMomentIso( date )
+							parseMomentIso( date, true )
 						);
 						changeImmediate( formattedDate, true );
 					} }

packages/js/components/src/date-time-picker-control/test/index.tsx

@@ -106,6 +106,46 @@ describe( 'DateTimePickerControl', () => {
 		);
 	} );
 
+	it( 'should assume ambiguous dates are UTC', () => {
+		const ambiguousISODateTimeString = '2202-09-15T22:30:40';
+
+		const { container } = render(
+			<DateTimePickerControl
+				currentDate={ ambiguousISODateTimeString }
+				is12Hour={ false }
+			/>
+		);
+
+		const input = container.querySelector( 'input' );
+
+		expect( input?.value ).toBe(
+			moment
+				.utc( ambiguousISODateTimeString )
+				.local()
+				.format( default24HourDateTimeFormat )
+		);
+	} );
+
+	it( 'should handle unambiguous UTC dates', () => {
+		const unambiguousISODateTimeString = '2202-09-15T22:30:40Z';
+
+		const { container } = render(
+			<DateTimePickerControl
+				currentDate={ unambiguousISODateTimeString }
+				is12Hour={ false }
+			/>
+		);
+
+		const input = container.querySelector( 'input' );
+
+		expect( input?.value ).toBe(
+			moment
+				.utc( unambiguousISODateTimeString )
+				.local()
+				.format( default24HourDateTimeFormat )
+		);
+	} );
+
 	it( 'should use the default 12 hour date time format', () => {
 		const dateTime = moment( '2022-09-15 02:30:40' );