* @param {Date|Number} date - the date to be changed
* @param {Number} amount - the amount of days to be added. Positive decimals will be rounded using `Math.floor`, decimals less than zero will be rounded using `Math.ceil`.
* @returns {Date} - the new date with the days added
* @throws {TypeError} - 2 arguments required
*
* @example
* // Add 10 days to 1 September 2014:
* const result = addDays(new Date(2014, 8, 1), 10)