fix: fixed dateBlockTimingInTimezoneFunction()

- fixed issue where the startsAt time was not being evaluated with a proper date normal, causing Asia/Tokyo (and similar date ranges) evaluations to return a different start date than other timezones.

packages/date/src/lib/date/date.block.spec.ts

@@ -330,7 +330,7 @@ describe('dateBlockTimingFromDateRangeAndEvent()', () => {
 
     function describeTestsForTimezone(timezone: TimezoneString) {
       const startOfTodayInTimezone = dateTimezoneUtcNormal({ timezone }).systemDateToTargetDate(startOfDay(new Date()));
-      const timing = dateBlockTiming({ startsAt: addHours(startOfTodayInTimezone, 3), duration: 60 }, 2); // 2 days
+      const timing = dateBlockTiming({ startsAt: addHours(startOfTodayInTimezone, 3), duration: 60 }, 1); // 1 day
 
       describe(`${timezone}`, () => {
         it('should return a copy of a timing.', () => {
@@ -444,6 +444,19 @@ describe('dateBlockTimingInTimezoneFunction()', () => {
         expect(result.startsAt).toBeSameSecondAs(startsAt);
         expect(result.duration).toBe(duration);
       });
+
+      it('should create a timing in the afteroon in the UTC timezone.', () => {
+        const duration = 60;
+        const startsAt = addHours(startOfToday, 12);
+        const result = fn({ startsAt, duration }, 2);
+
+        const { start } = result;
+        const utcHours = start.getUTCHours();
+        expect(utcHours).toBe(utcTimezoneOffsetInHours);
+
+        expect(result.startsAt).toBeSameSecondAs(startsAt);
+        expect(result.duration).toBe(duration);
+      });
     });
 
     describe('America/Denver', () => {
@@ -464,6 +477,19 @@ describe('dateBlockTimingInTimezoneFunction()', () => {
         expect(result.startsAt).toBeSameSecondAs(startsAt);
         expect(result.duration).toBe(duration);
       });
+
+      it('should create a timing in the afteroon in the America/Denver timezone.', () => {
+        const duration = 60;
+        const startsAt = addHours(startOfToday, 12);
+        const result = fn({ startsAt, duration }, 2);
+
+        const { start } = result;
+        const utcHours = start.getUTCHours();
+        expect(utcHours).toBe(denverTimezoneOffsetInHours);
+
+        expect(result.startsAt).toBeSameSecondAs(startsAt);
+        expect(result.duration).toBe(duration);
+      });
     });
   });
 });

packages/date/src/lib/date/date.block.ts

@@ -375,6 +375,8 @@ function _dateBlockTimingFromDateBlockTimingStartEndDayDateRange(dateBlockTiming
 /**
  * Returns a copy of the input timing with the start time timezone in the given timezone.
  *
+ * The start time is a normal, and should still refer to the same UTC date, but with the given timing's offset.
+ *
  * @param timing
  */
 export type ChangeTimingToTimezoneFunction = (<T extends DateRangeStart>(timing: T) => T) & {
@@ -559,6 +561,15 @@ export function getRelativeDateForDateBlockTiming(timing: DateBlockTimingStart,
  */
 export type DateBlockTimingRangeInput = Pick<DateRangeDayDistanceInput, 'distance'> | DateRange | number;
 
+export interface DateBlockTimingOptions {
+  /**
+   * Timezone to evaluate the startsAt time in.
+   *
+   * Will convert the input startsAt time to a normal in the given timezone, then converts it back to the system timezone.
+   */
+  timezone?: DateTimezoneUtcNormalFunctionInput;
+}
+
 /**
  * Creates a valid DateBlock timing from the DateDurationSpan and range input.
  *
@@ -573,14 +584,20 @@ export type DateBlockTimingRangeInput = Pick<DateRangeDayDistanceInput, 'distanc
  *
  * The start date from the inputDate is considered to to have the offset noted in DateBlock, and will be retained.
  */
-export function dateBlockTiming(durationInput: DateDurationSpan, inputRange: DateBlockTimingRangeInput): DateBlockTiming {
+export function dateBlockTiming(durationInput: DateDurationSpan, inputRange: DateBlockTimingRangeInput, options?: DateBlockTimingOptions): DateBlockTiming {
   const { duration } = durationInput;
+  const { timezone: timezoneInput } = options ?? {};
+  const timezoneInstance = timezoneInput ? dateTimezoneUtcNormal(timezoneInput) : undefined;
 
   if (duration > MINUTES_IN_DAY) {
     throw new Error('dateBlockTiming() duration cannot be longer than 24 hours.');
   }
 
-  let { startsAt } = durationInput;
+  let { startsAt: inputStartsAt } = durationInput;
+
+  // it is important that startsAt is evaluated the system time normal, as addDays/addMinutes and related functionality rely on the system timezone.
+  let startsAt = timezoneInstance ? timezoneInstance.systemDateToTargetDate(inputStartsAt) : inputStartsAt;
+
   let numberOfBlockedDays: number;
 
   let inputDate: Date | undefined;
@@ -620,6 +637,7 @@ export function dateBlockTiming(durationInput: DateDurationSpan, inputRange: Dat
   }
 
   const start = range.start;
+  startsAt = timezoneInstance ? timezoneInstance.targetDateToSystemDate(startsAt) : startsAt;
 
   // calculate end to be the ending date/time of the final duration span
   const lastStart = addDays(startsAt, numberOfBlockedDays);
@@ -644,7 +662,7 @@ export function dateBlockTimingInTimezoneFunction(input: TimingDateTimezoneUtcNo
   const changeTimezoneFunction = changeTimingToTimezoneFunction(input);
 
   const fn = ((durationInput: DateDurationSpan, inputRange: DateBlockTimingRangeInput) => {
-    const timing = dateBlockTiming(durationInput, inputRange);
+    const timing = dateBlockTiming(durationInput, inputRange, { timezone: changeTimezoneFunction._timezoneInstance });
     return changeTimezoneFunction(timing);
   }) as Building<DateBlockTimingInTimezoneFunction>;
 

packages/date/src/lib/date/date.schedule.spec.ts

@@ -1,4 +1,4 @@
-import { DateBlockIndex, dateBlockTimingInTimezone, timingIsInExpectedTimezone } from './date.block';
+import { DateBlockIndex, dateBlockTimingInTimezone, timingDateTimezoneUtcNormal, timingIsInExpectedTimezone } from './date.block';
 import { DateBlock, dateBlockTiming, systemNormalDateToBaseDate, DateScheduleRange, startOfDayInTimezoneDayStringFactory, startOfDayInTimezoneFromISO8601DayString } from '@dereekb/date';
 import {
   expandDateScheduleFactory,
@@ -650,9 +650,6 @@ describe('dateBlockTimingForExpandDateScheduleRangeInput()', () => {
 
         expect(result.start).toBeSameSecondAs(timing.start);
         expect(result.end).toBeSameSecondAs(timing.end);
-
-        const daysCount = differenceInDays(result.end, result.start);
-        expect(daysCount).toBe(days);
       });
     });
 
@@ -662,7 +659,7 @@ describe('dateBlockTimingForExpandDateScheduleRangeInput()', () => {
       const startsAt = addHours(startOfDay, 12); // Noon on 2022-01-02 in America/New_York
       const timing = dateBlockTimingInTimezone({ startsAt, duration: 30 }, 1, timezone);
 
-      it('should generate a valid DateBlockTiming() with the new duration.', () => {
+      it('should generate a valid DateBlockTiming with the new duration and same duration.', () => {
         expect(timingIsInExpectedTimezone(timing, { timezone })).toBe(true);
 
         const newDuration = 60;
@@ -684,7 +681,7 @@ describe('dateBlockTimingForExpandDateScheduleRangeInput()', () => {
         expect(result.end).toBeSameSecondAs(addMinutes(timing.end, durationDifference)); // 30 minutes later
       });
 
-      it('should generate a valid DateBlockTiming() with the same event startsAt time.', () => {
+      it('should generate a valid DateBlockTiming with the same event startsAt time.', () => {
         const newStartsAt = addDays(timing.startsAt, 1); // same event schedule
 
         const result = dateBlockTimingForExpandDateScheduleRangeInput({
@@ -702,7 +699,7 @@ describe('dateBlockTimingForExpandDateScheduleRangeInput()', () => {
         expect(result.end).toBeSameSecondAs(timing.end);
       });
 
-      it('should generate a valid DateBlockTiming() with the new duration and start at time.', () => {
+      it('should generate a valid DateBlockTiming with the new duration and start at time.', () => {
         const newDuration = 45;
         const hoursDifference = 1;
         const expectedStartsAt = addHours(timing.startsAt, hoursDifference);

packages/date/src/lib/date/date.timezone.spec.ts

@@ -225,5 +225,28 @@ describe('startOfDayInTimezoneDayStringFactory()', () => {
         expect(result.toISOString()).not.toBe(utcDateString);
       });
     });
+
+    describe('America/New_York', () => {
+      const timezone = 'America/New_York';
+      const fn = startOfDayInTimezoneDayStringFactory(timezone);
+      const instance = new DateTimezoneUtcNormalInstance({ timezone });
+
+      const expectedDay: ISO8601DayString = `2023-03-12`;
+      const utcDateString = `${expectedDay}T00:00:00.000Z`;
+      const utcStart = new Date(utcDateString);
+
+      const systemStart = instance.systemDateToBaseDate(utcStart);
+      const expectedStart = instance.targetDateToBaseDate(utcStart);
+
+      it('should return the start of the day date in America/New_York.', () => {
+        const inputDayString = formatToISO8601DayString(systemStart); // format to ensure that the same day is being passed
+        expect(expectedDay).toBe(inputDayString);
+        expect(systemStart).toBeSameSecondAs(startOfDay(systemStart));
+
+        const result = fn(inputDayString);
+        expect(result).toBeSameSecondAs(expectedStart);
+        expect(result.toISOString()).not.toBe(utcDateString);
+      });
+    });
   });
 });