* @param {Date|Number} date - the date to be changed
* @param {Number} amount - the amount of months to be subtracted. 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 months subtracted
* @throws {TypeError} 2 arguments required
*
* @example
* // Subtract 5 months from 1 February 2015:
* const result = subMonths(new Date(2015, 1, 1), 5)