daysBetween

1/**
2 * Returns the number of full days between two dates.
3 *
4 * @param date1 - The first date.
5 * @param date2 - The second date.
6 * @returns Number of days between the two dates (absolute value).
7 */
8export function daysBetween(date1: Date, date2: Date): number {
9  const msPerDay = 1000 * 60 * 60 * 24;
10  const utc1 = Date.UTC(date1.getFullYear(), date1.getMonth(), date1.getDate());
11  const utc2 = Date.UTC(date2.getFullYear(), date2.getMonth(), date2.getDate());
12
13  return Math.abs(Math.floor((utc2 - utc1) / msPerDay));
14}

1test('daysBetween - same date', () => {
2  const d = new Date('2023-01-01');
3  expect(daysBetween(d, d)).toBe(0);
4});
5
6test('daysBetween - one day apart', () => {
7  const d1 = new Date('2023-01-01');
8  const d2 = new Date('2023-01-02');
9  expect(daysBetween(d1, d2)).toBe(1);
10});
11
12test('daysBetween - multiple days apart', () => {
13  const d1 = new Date('2023-01-01');
14  const d2 = new Date('2023-01-10');
15  expect(daysBetween(d1, d2)).toBe(9);
16});
17
18test('daysBetween - order does not matter', () => {
19  const d1 = new Date('2023-01-10');
20  const d2 = new Date('2023-01-01');
21  expect(daysBetween(d1, d2)).toBe(9);
22});
23
24test('daysBetween - accounts for time differences', () => {
25  const d1 = new Date('2023-01-01T23:59:59');
26  const d2 = new Date('2023-01-02T00:00:01');
27  expect(daysBetween(d1, d2)).toBe(1);
28});