Javascript get number of days since start of Gregorian calendar

gregorianDateInteger.mjs

/*
 * Calculate the number of days since the start of the Gregorian calendar.
 * @param {Date} [date] - The reference date. Defaults to the current instant.
 * @param {boolean} [useLocalTime] - Whether to calculate in local time. WARNING: By default, UTC is used; this will, for instance, give a Tuesday result when run at 10pm on a Monday in the United States.
 * @returns {number}
 */
export function gregorianDateInteger(d, useLocalTime) {
	if (d === undefined) d = new Date();
	if (useLocalTime === undefined) useLocalTime = false;

	// 1. Get timestamp
	// 2. Apply timezone tweak, if applicable (do this in the integer realm to avoid floating-point edge cases)
	// 3. Convert to fractional days
	// 4. Truncate to whole days since 1970-01-01
	// 5. Adjust to whole days since 1582-10-14
	return Math.floor((d.valueOf() + (useLocalTime ? (d.getTimezoneOffset() * (-1000*60)) : 0)) / (1000*60*60*24)) + 141428;
}

export default gregorianDateInteger;