Create Gregorian to JDN conversion function

src/Calendar.test.ts

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+import {gregorianJDN} from "./Calendar";
+
+describe('gregorianJDN', () => {
+    it('works', () => {
+        expect(gregorianJDN(2000, 1, 1)).toBe(2451545);
+        expect(gregorianJDN(-4713, 11, 24)).toBe(0);
+        expect(gregorianJDN(11917, 9, 18)).toBe(6073915);
+        expect(gregorianJDN(-28565, 6, 17)).toBe(-8711925);
+        expect(gregorianJDN(-26650, 4, 13)).toBe(-8012550);
+        expect(gregorianJDN(17430, 3, 8)).toBe(8087303);
+        expect(gregorianJDN(3395, 7, 18)).toBe(2961257);
+        expect(gregorianJDN(4579, 3, 11)).toBe(3393575);
+        expect(gregorianJDN(-14851, 11, 22)).toBe(-3702831);
+        expect(gregorianJDN(8824, 11, 28)).toBe(4944292);
+        expect(gregorianJDN(19720, 8, 14)).toBe(8923868);
+        expect(gregorianJDN(7504, 7, 22)).toBe(4462042);
+    });
+});

src/Calendar.ts

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+export function gregorianJDN(year: number, month: number, day: number) {
+    const g = year + 4716 - (month <= 2 ? 1 : 0);
+    const f = (month + 9) % 12;
+    const e = Math.floor(1461 * g / 4) + day - 1402;
+    const J = e + Math.floor((153 * f + 2) / 5);
+    const dg = 38 - Math.floor(Math.floor((g + 184) / 100) * 3 / 4);
+    return J + dg;
+}