Add JDN to Gregorian date converter

src/dates.test.ts

@@ -1,4 +1,4 @@
-import {dateName, frJDN, gregorianJDN, monthName} from './dates';
+import {dateName, frJDN, gregorianJDN, jdnGregorian, monthName} from './dates';
 
 describe('gregorianJDN', () => {
     it('works', () => {
@@ -30,6 +30,27 @@ describe('frJDN', () => {
     });
 });
 
+describe('jdnGregorian', () => {
+    it('works', () => {
+        expect(jdnGregorian(0)).toEqual(new Date(-4713, 11, 24));
+        expect(jdnGregorian(2299160)).toEqual(new Date(1582, 10, 14));
+        expect(jdnGregorian(2299161)).toEqual(new Date(1582, 10, 15));
+        expect(jdnGregorian(2361221)).toEqual(new Date(1752, 9, 13));
+        expect(jdnGregorian(2361222)).toEqual(new Date(1752, 9, 14));
+        expect(jdnGregorian(2451545)).toEqual(new Date(2000, 1, 1));
+        expect(jdnGregorian(-8512316)).toEqual(new Date(-28019, 12, 20));
+        expect(jdnGregorian(-8534852)).toEqual(new Date(-28080, 4, 8));
+        expect(jdnGregorian(2653462)).toEqual(new Date(2552, 10, 30));
+        expect(jdnGregorian(3271156)).toEqual(new Date(4244, 1, 8));
+        expect(jdnGregorian(-666477)).toEqual(new Date(-6537, 2, 23));
+        expect(jdnGregorian(2397854)).toEqual(new Date(1852, 12, 31));
+        expect(jdnGregorian(-1211235)).toEqual(new Date(-8029, 8, 26));
+        expect(jdnGregorian(-91680)).toEqual(new Date(-4964, 11, 20));
+        expect(jdnGregorian(-5605876)).toEqual(new Date(-20061, 7, 14));
+        expect(jdnGregorian(-295121)).toEqual(new Date(-5521, 11, 19));
+    });
+});
+
 describe('monthName', () => {
     it('works', () => {
         expect(monthName(1)).toBe('Vendémiaire');

src/dates.ts

@@ -34,6 +34,12 @@ export type Day =
     | 29
     | 30;
 
+export type GregorianDate = {
+    year: number,
+    month: number,
+    day: number,
+};
+
 const monthNames: {
     [key in Month]: string
 } = {
@@ -81,6 +87,15 @@ export function frJDN(year: number, month: Month, day: Day): number {
     return startJD + 365 * dy + leaps[dy] + dd;
 }
 
+export function jdnGregorian(jdn: number): Date {
+    const e = 4 * (jdn + 1401 + Math.floor(Math.floor((4 * jdn + 274277) / 146097) * 3 / 4) - 38) + 3;
+    const h = 5 * Math.floor((e % 1461 + 1461) % 1461 / 4) + 2;
+    const day = Math.floor((h % 153 + 153) % 153 / 5) + 1;
+    const month = (Math.floor(h / 153) + 2) % 12 + 1;
+    const year = Math.floor(e / 1461) - 4716 + Math.floor((14 - month) / 12);
+    return new Date(year, month, day);
+}
+
 export function monthName(month: Month): string {
     return monthNames[month];
 }