Date Calculator

What this calculator does

Give it a starting date and an amount of time, and it tells you the date you land on. It works both ways: forwards for "what date is 90 days from today", backwards for "what date was six weeks ago". The result comes with the day of the week, because that is usually the next thing you need to know.

Calendar days or business days

These are different questions and the calculator treats them as different modes rather than as a checkbox.

• Calendar days count every day. Thirty days from a Friday is a Sunday, and that is the correct answer when a contract says "30 days".
• Business days step over weekends and public holidays as they go. Ten business days from a Friday is two weeks the following Friday, later still if a holiday falls in between.

Choosing business days switches off the weeks, months and years boxes. That is deliberate: "three business months" is not a quantity anybody means, and a field that silently contributed nothing would be worse than one that is plainly unavailable.

How adding months works

Months are not a fixed number of days, so adding one has to make a decision at the end of the month. The rule used here is the one every date library and every contract uses: the day of the month is kept, and clamped down when the target month is shorter. So 31 January plus one month is 28 February, and 31 March minus one month is 28 February in a normal year, 29 February in a leap year.

When you combine units, the largest is applied first - years, then months, then weeks and days. This matters more than it sounds. "One month and 10 days from 31 January" is 28 February plus 10 days, which is 10 March. Applying the days first would give 10 February plus a month, which is also 10 March in this case but not in others. The calculator prints the order it used, so the answer can be checked rather than trusted.

Leap years

Leap days are handled by the calendar itself rather than by a rule bolted on top, so a span that crosses 29 February simply includes it. Adding a year to 29 February gives 28 February in the following, non-leap year - the same clamping rule as months.

Frequently asked questions

What date will it be in 90 days?

Enter today as the starting date, choose Add, and put 90 in the days box. The result and its weekday appear immediately.

How do I subtract days from a date?

Switch the direction to Subtract. Everything else works the same way, and the working shown below the result describes the subtraction rather than an addition.

Does it count the starting date?

No. "Three days from Monday" means Thursday, not Wednesday - the count starts the day after. This is the reading used everywhere in contracts and shipping, and it is what the business-day mode uses too.

What about weekends and holidays?

In calendar mode they are counted like any other day. In business-day mode they are skipped, using the public holidays of whichever country you select.

Related date tools

Use days between dates to go the other way and measure a gap, the business days calculator to count working days in a range, week numbers for ISO weeks, and the meeting time planner for scheduling across time zones.