Add or Subtract Days From a Date

How to use Date Add & Subtract Calculator

1. 1
   Set the starting date. Enter it as a calendar date. Everything is computed in UTC so the answer does not shift with your timezone.
2. 2
   Choose an amount and unit. Days, working days, weeks, months or years — and whether it falls before or after the starting date.
3. 3
   Read the result. The date, the weekday it lands on, and how many calendar and working days separate the two.

About adding time to a date

Date arithmetic looks trivial and contains two traps that produce real bugs. The first is that months are not a fixed length, so adding one month is not adding thirty days. Every sensible implementation clamps: 31 January plus a month is the last day of February, because there is no 31 February and rolling forward into March answers a question nobody asked. The consequence is that month arithmetic is not reversible — adding a month to 31 January and then subtracting one gives 28 January, not 31 — which matters anywhere a date is computed by chaining operations rather than in a single step. The second trap is the timezone. A date with no time attached has no timezone to honour, but a naive implementation attaches local midnight, and a visitor several hours behind the machine that computed it sees the previous day. Everything here works in UTC for that reason. Working-day counting adds a third consideration: whether the starting day counts. The convention used here, and in most legal drafting, is that a period begins the day after the triggering event, so five working days after a Monday is the following Monday rather than that Friday. Public holidays are deliberately excluded, since they vary by country and several move by more than a month between years.

Frequently asked questions