fix: correct tests for createAppointment

Corrects the test so that they respect locales with Daylight Savings Time.

Previously, tests and suggested implementation simply did simple time arithmetic based on the offset
provided to `createAppointment`. This is wrong, as it will shift appointment time-of-day when moving
accross DST boundaries. This changes test implementation so that:
1. correct usage of input times is checked, by passing a 0 offset
2. correct offsetting of appointment time is checked, by passing in a known start date and then
creating one appointment that is within the same DST state and one that is not.

re: https://forum.exercism.org/t/tests-and-suggested-implementation-for-appointment-time-are-wrong/

Author: BadIdeaException

exercises/concept/appointment-time/appointment-time.spec.js

@@ -10,34 +10,38 @@ import {
 } from './appointment-time';
 
 describe('createAppointment', () => {
-  test('creates appointment 4 days in the future', () => {
-    const currentTime = Date.now();
-    const expectedTime = currentTime + 345600 * 1000;
+  test('uses the passed in current time', () => {
+    const currentTime = Date.UTC(2000, 6, 16, 12, 0, 0, 0);
+    const result = createAppointment(0, currentTime);
 
-    expect(createAppointment(4, currentTime)).toEqual(new Date(expectedTime));
+    expect(result).toEqual(new Date(currentTime));
   });
 
-  test('creates appointment 124 in the future', () => {
+  test('uses the actual current time when it is not passed in', () => {
     const currentTime = Date.now();
-    const expectedTime = currentTime + 10713600 * 1000;
+    const result = createAppointment(0);
 
-    expect(createAppointment(124, currentTime)).toEqual(new Date(expectedTime));
+    expect(result).toEqual(new Date(currentTime));
   });
 
-  test('uses the passed in current time', () => {
-    const currentTime = Date.UTC(2000, 6, 16, 12, 0, 0, 0);
-    const result = createAppointment(0, currentTime);
+  test('creates appointment without DST change', () => {
+    const offset = 4; // days
 
-    expect(result.getFullYear()).toEqual(2000);
+    const currentTime = Date.UTC(2000, 6, 1, 12, 0, 0, 0);
+    const expectedTime = currentTime + offset * 24 * 60 * 60 * 1000;
+
+    expect(createAppointment(offset, currentTime)).toEqual(new Date(expectedTime));
   });
 
-  test('uses the actual current time when it is not passed in', () => {
-    const result = createAppointment(0);
+  test('creates appointment with potential DST change', () => {
+    const offset = 180; // days
 
-    expect(Math.abs(Date.now() - result.getTime())).toBeLessThanOrEqual(
-      // Maximum number of time zones difference
-      27 * 60 * 60 * 1000,
-    );
+    const currentTime = Date.UTC(2000, 6, 1, 12, 0, 0, 0);
+    let expectedTime = currentTime + offset * 24 * 60 * 60 * 1000;
+    // Manually adjust for DST timezone offset:
+    expectedTime += (new Date(expectedTime).getTimezoneOffset() - new Date(currentTime).getTimezoneOffset()) * 60 * 1000;
+
+    expect(createAppointment(offset, currentTime)).toEqual(new Date(expectedTime));
   });
 
   test('rolls over days, months, and years', () => {