when-am-i / files / src / utils / julianDay.test.ts

julianDay.test.ts

1	import { describe, it, expect } from 'vitest';
2	import {
3	gregorianToJulianDay,
4	julianDayToGregorian,
5	julianDayToJulian,
6	parseGregorianDate,
7	} from './julianDay';
8
9	describe('gregorianToJulianDay', () => {
10	it('matches the J2000 epoch (2000-01-01 = JDN 2451545)', () => {
11	expect(gregorianToJulianDay({ year: 2000, month: 1, day: 1 })).toBe(2451545);
12	});
13
14	it('orders dates monotonically across the BCE/CE boundary', () => {
15	const bce = gregorianToJulianDay({ year: -1, month: 12, day: 31 }); // 2 BCE
16	const ce = gregorianToJulianDay({ year: 1, month: 1, day: 1 }); // 1 CE
17	expect(bce).toBeLessThan(ce);
18	});
19	});
20
21	describe('julianDayToGregorian', () => {
22	it('round-trips with the forward conversion', () => {
23	for (const date of [
24	{ year: 2000, month: 1, day: 1 },
25	{ year: 1582, month: 10, day: 15 },
26	{ year: 1, month: 1, day: 1 },
27	{ year: -44, month: 3, day: 15 },
28	]) {
29	expect(julianDayToGregorian(gregorianToJulianDay(date))).toEqual(date);
30	}
31	});
32	});
33
34	describe('julianDayToJulian', () => {
35	it('2000-01-01 Gregorian is 1999-12-19 in the Julian calendar (13-day gap)', () => {
36	const jdn = gregorianToJulianDay({ year: 2000, month: 1, day: 1 });
37	expect(julianDayToJulian(jdn)).toEqual({ year: 1999, month: 12, day: 19 });
38	});
39
40	it('1582-10-15 Gregorian is 1582-10-05 Julian (the 10-day reform gap)', () => {
41	const jdn = gregorianToJulianDay({ year: 1582, month: 10, day: 15 });
42	expect(julianDayToJulian(jdn)).toEqual({ year: 1582, month: 10, day: 5 });
43	});
44	});
45
46	describe('parseGregorianDate', () => {
47	it('parses a normal date', () => {
48	expect(parseGregorianDate('1750-01-01')).toEqual({ year: 1750, month: 1, day: 1 });
49	});
50
51	it('parses a negative (BCE) year', () => {
52	expect(parseGregorianDate('-0044-03-15')).toEqual({ year: -44, month: 3, day: 15 });
53	});
54
55	it('throws on a malformed string', () => {
56	expect(() => parseGregorianDate('not-a-date')).toThrow();
57	});
58	});