[Calculate new Date off Date] #date

delta-date-fns.js

function deltaDate(input, days, months, years) {
  return new Date(
    input.getFullYear() + years,
    input.getMonth() + months,
    Math.min(
      input.getDate() + days,
      new Date(
        input.getFullYear() + years,
        input.getMonth() + months + 1,
        0
      ).getDate()
    )
  );
}

function deltaDateMomentJs(input, amount, dateType) {
  const dt = new Date(input);
  switch (dateType) {
    case 'days':
      return dt.setDate(dt.getDate() + amount) && dt;
    case 'weeks':
      return dt.setDate(dt.getDate() + 7 * amount) && dt;
    case 'months':
      return dt.setMonth(dt.getMonth() + amount) && dt;
    case 'years':
      return dt.setFullYear(dt.getFullYear() + amount) && dt;
  }
}