Add asUTC option to getMonthFromDate util

plus new tests to verify the behavior

packages/date-picker/src/utils.test.ts

@@ -23,6 +23,16 @@ describe('@acusti/date-picker', () => {
                 expect(getMonthFromDate(new Date(1968, 3, 30))).toBe(-21); // april 30, 1968
             });
 
+            it('returns the correct month digit based on UTC time if asUTC is true', () => {
+                const date = new Date('1970-01-01T00:00:00.000Z');
+                expect(getMonthFromDate(date, true)).toBe(0);
+                // if test is run in a timezone behind UTC, ommitting asUTC returns -1
+                // if not, the month should be the same with or without asUTC flag
+                expect(getMonthFromDate(date)).toBe(
+                    date.getTimezoneOffset() >= 60 ? -1 : 0,
+                );
+            });
+
             it('returns NaN for an Invalid Date', () => {
                 expect(getMonthFromDate(INVALID_DATE)).toBe(NaN);
             });

packages/date-picker/src/utils.ts

@@ -16,10 +16,13 @@ const MONTH_NAMES = [
     'December',
 ];
 
-const getYearFromDate = (date: Date) => date.getFullYear() - START_YEAR;
+const getYearFromDate = (date: Date, asUTC?: boolean) =>
+    (asUTC ? date.getUTCFullYear() : date.getFullYear()) - START_YEAR;
 
-export const getMonthFromDate = (date: Date) =>
-    date.getMonth() + (getYearFromDate(date) * 12); // prettier-ignore
+export const getMonthFromDate = (date: Date, asUTC?: boolean) => {
+    const yearAsMonths = getYearFromDate(date, asUTC) * 12;
+    return yearAsMonths + (asUTC ? date.getUTCMonth() : date.getMonth());
+};
 
 export const getYearFromMonth = (month: number) => Math.floor(month / 12) + START_YEAR;