Date & time
Date calculator (add or subtract)
Move forwards or backwards from a date by any combination of days, weeks, months and years — or count working days only, which is how most deadlines actually run.
Optional, used only in working-days mode.
Adding months is not adding days
Every date library in the world has to make the same awkward decision, and this one is no exception. Adding a month means landing on the same day number in the next month. When that day does not exist, something has to give.
The near-universal convention, and the one used here, is to clamp to the last day of the target month. 31 January plus one month is 28 February, or 29 February in a leap year. The alternative — rolling over into 3 March — produces stranger results, such as a monthly subscription that drifts later every year.
Note that this makes the operation asymmetric: adding a month and then subtracting one does not always return you to where you started. That is not a bug in the arithmetic, it is a consequence of months having different lengths.
Order of operations matters
This calculator applies years and months first, then weeks and days. Doing it the other way round can give a different answer. Starting from 30 January, adding one day then one month gives 28 February, but adding one month then one day gives 1 March. Months-first is the standard order and the one that matches how people describe dates.
Working-day arithmetic
Contracts, courts and shipping estimates usually count business days, not calendar days. “Payment within 30 days” and “payment within 30 working days” are six weeks apart in practice.
| Working days | Roughly equals |
|---|---|
| 5 | 1 calendar week |
| 10 | 2 calendar weeks |
| 21 | 1 calendar month |
| 65 | 1 calendar quarter |
| 261 | 1 calendar year |
In working-days mode, months and years are converted at 21 and 261 days respectively, since there is no such thing as a “working month” on the calendar.
ISO week numbers
The result shows the ISO 8601 week number, which is what most European businesses and manufacturing schedules run on. Week 1 is the week containing the first Thursday of January, which means early January can fall in week 52 or 53 of the previous year. A year has 52 weeks, or 53 when 1 January is a Thursday, or a Wednesday in a leap year.
Common questions
What date is 90 days from today?
Set the start date to today, leave the direction on add, clear the other fields and enter 90 in the days box. Ninety calendar days is close to three months but not identical — it runs one to two days short depending on which months it crosses, since three months averages 91.3 days.
Why does adding one month to 31 August give 30 September?
September has only 30 days, so the 31st does not exist and the result is clamped to the last valid day. Every mainstream date system does this. The practical implication is that monthly billing anchored to the 29th, 30th or 31st will land on the last day of short months.
Do working days include public holidays?
Only if you leave the holiday field empty. Holidays differ so much between countries and even regions that guessing would be worse than asking, so any dates you list are skipped along with weekends. Holidays that already fall on a weekend make no difference to the count.
How do I find the date a certain number of days ago?
Switch the direction to subtract. The count runs backwards using the same rules, so subtracting months clamps to month ends in the same way.