wip refactoring manual timezone offset calculations in date.pipe from native Date methods to momentjs

Author: chrisjones-hmcts

projects/ccd-case-ui-toolkit/src/lib/shared/components/palette/utils/date.pipe.spec.ts

@@ -162,6 +162,280 @@ describe('DatePipe', () => {
      (two hours behind or in front) when moving through the various steps.
   */
 
+  describe('British Summer Time (BST) Transitions - EXUI-3066', () => {
+    // BST starts: Last Sunday in March at 1:00 AM GMT → 2:00 AM BST (UTC+1)
+    // BST ends: Last Sunday in October at 2:00 AM BST → 1:00 AM GMT (UTC+0)
+
+    describe('Spring Forward - Clocks go forward (GMT to BST)', () => {
+      it('should correctly display UTC time just before spring forward transition', () => {
+        // March 26, 2023 at 00:30 UTC (30 mins before clocks spring forward)
+        const UTC_BEFORE_SPRING = '2023-03-26T00:30:00.000';
+        const result = datePipe.transform(UTC_BEFORE_SPRING, 'local', null);
+
+        // In local time (GMT), this would still be 00:30 AM on March 26
+        expect(result).toContain('26 Mar 2023');
+        expect(result).toContain('12:30:00 AM');
+      });
+
+      it('should correctly display UTC time just after spring forward transition', () => {
+        // March 26, 2023 at 01:30 UTC (30 mins after clocks spring forward)
+        // At 1:00 AM GMT, clocks jumped to 2:00 AM BST
+        const UTC_AFTER_SPRING = '2023-03-26T01:30:00.000';
+        const result = datePipe.transform(UTC_AFTER_SPRING, 'local', null);
+
+        // In local time (BST = UTC+1), 01:30 UTC should be 02:30 BST
+        expect(result).toContain('26 Mar 2023');
+        expect(result).toContain('2:30:00 AM');
+      });
+
+      it('should handle UTC time during the missing hour (1:00-2:00 AM GMT)', () => {
+        // March 26, 2023 at 01:15 UTC (during the non-existent hour in local time)
+        const UTC_MISSING_HOUR = '2023-03-26T01:15:00.000';
+        const result = datePipe.transform(UTC_MISSING_HOUR, 'local', null);
+
+        // This time doesn't exist in local time, but moment converts it to BST
+        // 01:15 UTC = 02:15 BST
+        expect(result).toContain('26 Mar 2023');
+        expect(result).toContain('2:15:00 AM');
+      });
+
+      it('should handle date-only value during spring forward day', () => {
+        // Date without time during spring forward should not shift
+        const SPRING_FORWARD_DATE = '2023-03-26';
+        const result = datePipe.transform(SPRING_FORWARD_DATE, null, null);
+
+        expect(result).toBe('26 Mar 2023');
+      });
+
+      it('should correctly handle noon UTC on spring forward day', () => {
+        // March 26, 2023 at 12:00 UTC (well after transition)
+        const UTC_NOON = '2023-03-26T12:00:00.000';
+        const result = datePipe.transform(UTC_NOON, 'local', null);
+
+        // 12:00 UTC = 13:00 BST (1:00 PM)
+        expect(result).toContain('26 Mar 2023');
+        expect(result).toContain('1:00:00 PM');
+      });
+    });
+
+    describe('Fall Back - Clocks go back (BST to GMT)', () => {
+      it('should correctly display UTC time just before fall back transition', () => {
+        // October 29, 2023 at 00:30 UTC (30 mins before clocks fall back)
+        const UTC_BEFORE_FALL = '2023-10-29T00:30:00.000';
+        const result = datePipe.transform(UTC_BEFORE_FALL, 'local', null);
+
+        // Still in BST, so 00:30 UTC = 01:30 BST
+        expect(result).toContain('29 Oct 2023');
+        expect(result).toContain('1:30:00 AM');
+      });
+
+      it('should correctly display UTC time just after fall back transition', () => {
+        // October 29, 2023 at 01:30 UTC (after clocks fell back)
+        // At 2:00 AM BST, clocks went back to 1:00 AM GMT
+        const UTC_AFTER_FALL = '2023-10-29T01:30:00.000';
+        const result = datePipe.transform(UTC_AFTER_FALL, 'local', null);
+
+        // Now in GMT, so 01:30 UTC = 01:30 GMT
+        expect(result).toContain('29 Oct 2023');
+        expect(result).toContain('1:30:00 AM');
+      });
+
+      it('should handle UTC time during the repeated hour (1:00-2:00 AM)', () => {
+        // October 29, 2023 at 01:15 UTC (during the hour that occurs twice)
+        const UTC_REPEATED_HOUR = '2023-10-29T01:15:00.000';
+        const result = datePipe.transform(UTC_REPEATED_HOUR, 'local', null);
+
+        // Moment will interpret this as GMT (after the transition)
+        expect(result).toContain('29 Oct 2023');
+        expect(result).toContain('1:15:00 AM');
+      });
+
+      it('should handle date-only value during fall back day', () => {
+        // Date without time during fall back should not shift
+        const FALL_BACK_DATE = '2023-10-29';
+        const result = datePipe.transform(FALL_BACK_DATE, null, null);
+
+        expect(result).toBe('29 Oct 2023');
+      });
+
+      it('should correctly handle noon UTC on fall back day', () => {
+        // October 29, 2023 at 12:00 UTC (well after transition)
+        const UTC_NOON = '2023-10-29T12:00:00.000';
+        const result = datePipe.transform(UTC_NOON, 'local', null);
+
+        // 12:00 UTC = 12:00 GMT (after fall back)
+        expect(result).toContain('29 Oct 2023');
+        expect(result).toContain('12:00:00 PM');
+      });
+    });
+
+    describe('Regular BST and GMT periods', () => {
+      it('should correctly display UTC time during summer (BST period)', () => {
+        // July 15, 2023 at 14:30 UTC (middle of summer, BST in effect)
+        const UTC_SUMMER = '2023-07-15T14:30:00.000';
+        const result = datePipe.transform(UTC_SUMMER, 'local', null);
+
+        // 14:30 UTC = 15:30 BST (3:30 PM)
+        expect(result).toContain('15 Jul 2023');
+        expect(result).toContain('3:30:00 PM');
+      });
+
+      it('should correctly display UTC time during winter (GMT period)', () => {
+        // January 15, 2023 at 14:30 UTC (middle of winter, GMT in effect)
+        const UTC_WINTER = '2023-01-15T14:30:00.000';
+        const result = datePipe.transform(UTC_WINTER, 'local', null);
+
+        // 14:30 UTC = 14:30 GMT (2:30 PM)
+        expect(result).toContain('15 Jan 2023');
+        expect(result).toContain('2:30:00 PM');
+      });
+
+      it('should correctly display UTC midnight during BST', () => {
+        // August 1, 2023 at 00:00 UTC (midnight during BST)
+        const UTC_MIDNIGHT_BST = '2023-08-01T00:00:00.000';
+        const result = datePipe.transform(UTC_MIDNIGHT_BST, 'local', null);
+
+        // 00:00 UTC = 01:00 BST (1:00 AM)
+        expect(result).toContain('1 Aug 2023');
+        expect(result).toContain('1:00:00 AM');
+      });
+
+      it('should correctly display UTC midnight during GMT', () => {
+        // December 1, 2023 at 00:00 UTC (midnight during GMT)
+        const UTC_MIDNIGHT_GMT = '2023-12-01T00:00:00.000';
+        const result = datePipe.transform(UTC_MIDNIGHT_GMT, 'local', null);
+
+        // 00:00 UTC = 00:00 GMT (midnight)
+        expect(result).toContain('1 Dec 2023');
+        expect(result).toContain('12:00:00 AM');
+      });
+
+      it('should correctly display UTC 23:00 during BST (crosses day boundary)', () => {
+        // July 15, 2023 at 23:30 UTC (late evening during BST)
+        const UTC_LATE_BST = '2023-07-15T23:30:00.000';
+        const result = datePipe.transform(UTC_LATE_BST, 'local', null);
+
+        // 23:30 UTC = 00:30 next day BST (crosses midnight)
+        expect(result).toContain('16 Jul 2023');
+        expect(result).toContain('12:30:00 AM');
+      });
+    });
+
+    describe('Edge cases with timezone offset notation', () => {
+      it('should handle UTC datetime with Z suffix during BST period', () => {
+        const UTC_WITH_Z = '2023-06-15T10:30:00.000Z';
+        const result = datePipe.transform(UTC_WITH_Z, 'local', null);
+
+        // 10:30 UTC = 11:30 BST
+        expect(result).toContain('15 Jun 2023');
+        expect(result).toContain('11:30:00 AM');
+      });
+
+      it('should handle UTC datetime with +00:00 offset during GMT period', () => {
+        const UTC_WITH_OFFSET = '2023-01-15T10:30:00.000+00:00';
+        const result = datePipe.transform(UTC_WITH_OFFSET, 'local', null);
+
+        // 10:30 UTC = 10:30 GMT
+        expect(result).toContain('15 Jan 2023');
+        expect(result).toContain('10:30:00 AM');
+      });
+    });
+
+    describe('UTC mode (should not convert to local time)', () => {
+      it('should keep UTC time as-is when zone is utc during BST period', () => {
+        const UTC_TIME = '2023-07-15T14:30:00.000';
+        const result = datePipe.transform(UTC_TIME, 'utc', null);
+
+        // Should remain 14:30, not converted to BST
+        expect(result).toContain('15 Jul 2023');
+        expect(result).toContain('2:30:00 PM');
+      });
+
+      it('should keep UTC time as-is when zone is utc during GMT period', () => {
+        const UTC_TIME = '2023-01-15T14:30:00.000';
+        const result = datePipe.transform(UTC_TIME, 'utc', null);
+
+        // Should remain 14:30
+        expect(result).toContain('15 Jan 2023');
+        expect(result).toContain('2:30:00 PM');
+      });
+    });
+  });
+
+  describe('Edge case tests - EXUI-3066', () => {
+    it('should correctly handle timezone offset notation (+01:00)', () => {
+      const DATE_WITH_OFFSET = '2023-07-15T14:30:00.000+01:00';
+      const result = datePipe.transform(DATE_WITH_OFFSET, 'local', null);
+
+      // 14:30 in +01:00 timezone = 13:30 UTC = 14:30 BST (in UK summer)
+      expect(result).toContain('15 Jul 2023');
+      expect(result).toContain('2:30:00 PM');
+    });
+
+    it('should correctly handle negative timezone offsets (-05:00)', () => {
+      const DATE_WITH_NEG_OFFSET = '2023-01-15T10:00:00.000-05:00';
+      const result = datePipe.transform(DATE_WITH_NEG_OFFSET, 'local', null);
+
+      // 10:00 in -05:00 (EST) = 15:00 UTC = 15:00 GMT (in UK winter)
+      expect(result).toContain('15 Jan 2023');
+      expect(result).toContain('3:00:00 PM');
+    });
+
+    it('should handle datetime values crossing day boundary when converting to local', () => {
+      const LATE_UTC = '2023-08-15T23:30:00.000';
+      const result = datePipe.transform(LATE_UTC, 'local', null);
+
+      // 23:30 UTC in summer = 00:30 next day BST
+      expect(result).toContain('16 Aug 2023');
+      expect(result).toContain('12:30:00 AM');
+    });
+
+    it('should handle early morning UTC times during winter', () => {
+      const EARLY_UTC_WINTER = '2023-01-15T00:30:00.000';
+      const result = datePipe.transform(EARLY_UTC_WINTER, 'local', null);
+
+      // 00:30 UTC in winter = 00:30 GMT (no change)
+      expect(result).toContain('15 Jan 2023');
+      expect(result).toContain('12:30:00 AM');
+    });
+
+    it('should handle early morning UTC times during summer (stays on same day)', () => {
+      const EARLY_UTC_SUMMER = '2023-07-15T00:30:00.000';
+      const result = datePipe.transform(EARLY_UTC_SUMMER, 'local', null);
+
+      // 00:30 UTC in summer = 01:30 BST (same day)
+      expect(result).toContain('15 Jul 2023');
+      expect(result).toContain('1:30:00 AM');
+    });
+
+    it('should correctly handle DST transition with explicit UTC offset', () => {
+      const DST_TRANSITION_WITH_OFFSET = '2023-03-26T01:00:00.000+00:00';
+      const result = datePipe.transform(DST_TRANSITION_WITH_OFFSET, 'local', null);
+
+      // 01:00 UTC with +00:00 = 02:00 BST (after transition)
+      expect(result).toContain('26 Mar 2023');
+      expect(result).toContain('2:00:00 AM');
+    });
+
+    it('should handle timezone-aware string when converting to UTC mode', () => {
+      const WITH_OFFSET = '2023-06-15T14:30:00.000+02:00';
+      const result = datePipe.transform(WITH_OFFSET, 'utc', null);
+
+      // 14:30 in +02:00 = 12:30 UTC
+      expect(result).toContain('15 Jun 2023');
+      expect(result).toContain('12:30:00 PM');
+    });
+
+    it('should handle datetime at exact DST transition moment', () => {
+      const EXACT_TRANSITION = '2023-03-26T01:00:00.000';
+      const result = datePipe.transform(EXACT_TRANSITION, 'local', null);
+
+      // At 1:00 AM GMT, clocks jump to 2:00 AM BST
+      expect(result).toContain('26 Mar 2023');
+      expect(result).toContain('2:00:00 AM');
+    });
+  });
+
   function getExpectedHour(hour): number {
     let expectedHour = hour + EXPECTED_OFFSET;
     if (expectedHour > 12) {