qcal/src/Calendar.ts

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;
}