Adds or subtracts date portions to the given date.

alterDate.js

/**
 * Adds or subtracts date portions to the given date and returns the new date.
 *
 * @param  {Object} options: It contains the date parts to add or remove, and can have the following properties:
 *         - {Date} date: if provided, this date will be affected, otherwise the current date will be used.
 *         - {number} minutes: minutes to add/subtract
 *         - {Number} hours: hours to add/subtract
 *         - {Number} days: days to add/subtract
 *         - {Number} months: months to add/subtract
 *         - {Number} years: years to add/subtract
 * @return {Date}
 */
function alterDate(options = {}) {
  const opt = Object.assign({}, options);
  const d = opt.date instanceof Date ? opt.date : new Date();
  if (+opt.minutes) d.setMinutes(d.getMinutes() + opt.minutes);
  if (+opt.hours) d.setHours(d.getHours() + opt.hours);
  if (+opt.days) d.setDate(d.getDate() + opt.days);
  if (+opt.months) d.setMonth(d.getMonth() + opt.months);
  if (+opt.years) d.setFullYear(d.getFullYear() + opt.years);
  return d;
}

test.js

alterDate({years: -6});
// expected:
// six years before the current date

alterDate({days: 8})
// expected:
// 8 days after the current date

alterDate({months: -2});
// expected:
// two months before the current date

alterDate({
  date: new Date("2017/03/06"),
  hours: -8
});
// expected:
// eight hours before the specified date (in CST)

var date = new Date("1995-12-17T03:24:59") // UTC
var offset = date.getTimezoneOffset() / 60;
var newDate = alterDate({date, hours: offset});

newDate.toISOString();
// expected (GMT +0):
// "1995-12-17T09:24:59.000Z"