Entrovix AI

Date Calculator — Add or Subtract Days

Add or subtract time from a date in calendar days, months and years — or in working days, skipping weekends and holidays, which a calendar cannot do.

Runs in your browser — nothing is uploaded

Start date and offset

Month-ends clamp: 31 January plus a month is 28 or 29 February, not 3 March

Date you land on

2026-09-01

A Tuesday

2026-08-02 → 2026-09-01

Added 30d.

Why use it

Built to be genuinely useful

Working-day arithmetic

'Ten working days from today' is how refunds and notice periods are phrased. This answers it.

Month-ends handled

31 January plus one month is 28 or 29 February, not 3 March. Clamping is what people mean.

Forwards or backwards

Work out a deadline, or work back from one to find the date you needed to start.

Free, no sign-up

No account, no usage cap, and no feature held back behind a paywall.

How it works

Three steps

  1. 1

    Pick the starting date.

  2. 2

    Choose calendar days or working days, and whether you are adding or subtracting.

  3. 3

    For working days, set your weekend pattern and paste any holidays.

Ten working days is not ten days

Refund windows, notice periods, government processing times and service commitments are almost all quoted in working days. Counting ten of them forward from a Monday lands you on the Monday a fortnight later, not on the Thursday of the same week — a four-day difference on a two-week deadline.

Add a public holiday in the middle and it moves again. This is genuinely awkward to do on a calendar and impossible to do in your head reliably, and it is the single thing a date calculator could usefully offer that most do not.

The result shows both figures: the date you land on, and how many calendar days that turned out to be. The gap between the two is usually the surprise.

Adding months is not adding thirty days

One month after 31 January is 28 February — or 29 in a leap year. Not 2 or 3 March, which is what adding 31 days gives you, and not an error, which is what naive date arithmetic sometimes produces.

The convention used here is clamping: if the target month has no such day, you land on its last day. This matches how contracts, subscriptions and rental periods are read, and how every established date library behaves.

One consequence worth knowing: adding a month and then subtracting one does not always return you to where you started. From 31 January you get to 28 February and back to 28 January. That is not a bug in the arithmetic, it is a property of calendars.

Working backwards from a deadline

Subtracting working days is how you find a start date. If something is due on the 30th and takes fifteen working days, you needed to begin on a date the calendar will not tell you.

The subtract mode counts backwards over the same weekend and holiday rules, so the answer accounts for the days nobody is going to be working on it.

FAQ

Questions people ask

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