gistfile1.txt

function addMonths(date: Date, months: number): Date {
  const newDate = new Date(date)  
const dayOfMonth = newDate.getDate()

  // The JS Date object supports date math by accepting out-of-bounds values for
  // month, day, etc. For example, new Date(2020, 1, 0) returns 31 Dec 2019 and
  // new Date(2020, 13, 1) returns 1 Feb 2021.  This is *almost* the behavior we
  // want except that dates will wrap around the end of a month, meaning that
  // new Date(2020, 13, 31) will return 3 Mar 2021 not 28 Feb 2021 as desired. So
  // we'll default to the end of the desired month by adding 1 to the desired
  // month and using a date of 0 to back up one day to the end of the desired
  // month.
  const endOfDesiredMonth = new Date(newDate.getTime())
  const monthsPlusOne = Math.abs(months + 1) * (Math.sign(months) !== 0 ? Math.sign(months) : 1)
  endOfDesiredMonth.setMonth(newDate.getMonth() + monthsPlusOne, 0)
  const daysInMonth = endOfDesiredMonth.getDate()
  if (dayOfMonth >= daysInMonth) {
    // If we're already at the end of the month, then this is the correct date
    // and we're done.

    // if we're going back over daylight savings, we find the time-zone difference
    // and adjust, so that we don't end up being one day off
    const timezoneDiff = date.getTimezoneOffset() - endOfDesiredMonth.getTimezoneOffset()
    endOfDesiredMonth.setMinutes(endOfDesiredMonth.getMinutes() + timezoneDiff)
    return endOfDesiredMonth
  } else {
    // Otherwise, we now know that setting the original day-of-month value won't
    // cause an overflow, so set the desired day-of-month. Note that we can't
    // just set the date of `endOfDesiredMonth` because that object may have had
    // its time changed in the unusual case where where a DST transition was on
    // the last day of the month and its local time was in the hour skipped or
    // repeated next to a DST transition.  So we use `date` instead which is
    // guaranteed to still have the original time.
    newDate.setFullYear(
      endOfDesiredMonth.getFullYear(),
      endOfDesiredMonth.getMonth(),
      dayOfMonth
    )

    // if we're going back over daylight savings, we find the time-zone difference
    // and adjust, so that we don't end up being one day off
    const timezoneDiff = date.getTimezoneOffset() - newDate.getTimezoneOffset()
    newDate.setMinutes(newDate.getMinutes() + timezoneDiff)
    return newDate
  }