fix(common): parse `YYYY-MM` strings as UTC dates (#40620)

In 2aba8b0 we fixed the DatePipe so that when it parsed date strings
that looked like `YYYY-MM-DD` it created a UTC date that was not
affected by the local timezone of the JavaScript engine.

This commit does the same for date strings of the form `YYYY-MM`
and `YYYY`.

Fixes #33944

PR Close #40620

packages/common/src/i18n/format_date.ts

@@ -718,7 +718,7 @@ export function toDate(value: string|number|Date): Date {
       return new Date(parsedNb);
     }
 
-    if (/^(\d{4}-\d{1,2}-\d{1,2})$/.test(value)) {
+    if (/^(\d{4}(-\d{1,2}(-\d{1,2})?)?)$/.test(value)) {
       /* For ISO Strings without time the day, month and year must be extracted from the ISO String
       before Date creation to avoid time offset and errors in the new Date.
       If we only replace '-' with ',' in the ISO String ("2015,01,01"), and try to create a new
@@ -726,7 +726,7 @@ export function toDate(value: string|number|Date): Date {
       If we leave the '-' ("2015-01-01") and try to create a new Date("2015-01-01") the timeoffset
       is applied.
       Note: ISO months are 0 for January, 1 for February, ... */
-      const [y, m, d] = value.split('-').map((val: string) => +val);
+      const [y, m = 1, d = 1] = value.split('-').map((val: string) => +val);
       return new Date(y, m - 1, d);
     }
 

packages/common/test/i18n/format_date_spec.ts

@@ -60,6 +60,8 @@ describe('Format date', () => {
 
   describe('formatDate', () => {
     const isoStringWithoutTime = '2015-01-01';
+    const isoStringWithoutTimeOrDate = '2015-01';
+    const isoStringWithoutTimeOrDateOrMonth = '2015-01';
     const defaultFormat = 'mediumDate';
     let date: Date;
 
@@ -231,6 +233,16 @@ describe('Format date', () => {
         expectDateFormatAs(isoStringWithoutTime, pattern, isoStringWithoutTimeFixtures[pattern]);
       });
 
+      Object.keys(isoStringWithoutTimeFixtures).forEach((pattern: string) => {
+        expectDateFormatAs(
+            isoStringWithoutTimeOrDate, pattern, isoStringWithoutTimeFixtures[pattern]);
+      });
+
+      Object.keys(isoStringWithoutTimeFixtures).forEach((pattern: string) => {
+        expectDateFormatAs(
+            isoStringWithoutTimeOrDateOrMonth, pattern, isoStringWithoutTimeFixtures[pattern]);
+      });
+
       const nightTime = new Date(2015, 5, 15, 2, 3, 1, 550);
       Object.keys(midnightCrossingPeriods).forEach(pattern => {
         expectDateFormatAs(nightTime, pattern, midnightCrossingPeriods[pattern]);