Convert a decimal date to either human readable date or UNIX timestamp

decimalDate.js

	function leapYear(year) {
		return ((year % 4 === 0) && (year % 100 !== 0)) || (year % 400 === 0);
	};

	/**
	 * Converts a decimal date, e.g. January 15th 2017 would be 14 / 365 + 2017 = 2017.0384 (days start at 0)
	 * @param decimalDate
	 */
	function convertDecimalDate(decimalDate) {
		const year = parseInt(decimalDate);
		const remainder = decimalDate - year;
		const daysPerYear = leapYear(year) ? 366 : 365;
		const miliseconds = remainder * daysPerYear * 24 * 60 * 60 * 1000;
		const yearDate = new Date(year, 0, 1);
		return new Date(yearDate.getTime() + miliseconds);
	}

	const date = convertDecimalDate(2017.90685);
	console.log(`date: ${date}`);

	const epochTimestamp = Math.round((new Date(date)).getTime() / 1000);
	console.log(`epochTimestamp: ${epochTimestamp}`);