Date calculator
| Month | Days | Leap year |
|---|---|---|
| January | 31 | 31 |
| February | 28 | 29 |
| March | 31 | 31 |
| April | 30 | 30 |
| May | 31 | 31 |
| June | 30 | 30 |
| July | 31 | 31 |
| August | 31 | 31 |
| September | 30 | 30 |
| October | 31 | 31 |
| November | 30 | 30 |
| December | 31 | 31 |
Date arithmetic comes in four shapes: moving forward or back from a date, measuring the gap between two, adding working days only, and looking up a date or a period. All of them are here. A hundred days from Thursday 1 January 2026 is Saturday 11 April; ten working days from a Monday is the Monday a fortnight later.
How to use the date calculator
The one rule worth carrying away is that days and months behave differently. A day count is a fixed number of days; a month keeps the day-of-month number and clamps when the target month is shorter. The difference stays invisible until a month-end date is involved, at which point it becomes the whole answer: 31 January plus one month is 28 February, and 28 February minus one month is 28 January, so the operation does not undo itself.
Fixed day counts exist partly to dodge that. Ninety-day payment terms and thirty-day returns windows are counted in days because nobody can argue about a day, and the price is a predictable drift through the week. Thirty days moves the weekday forward by two; ninety moves it by six, which lands you one weekday earlier. That is why a quarterly billing cycle creeps backwards through the week and sooner or later falls at a weekend. Adding 365 is the one case that looks safe and is not: it returns the same date only when no 29 February falls inside the span.
Working days are a third thing again. Ten of them from a Monday is the Monday a fortnight later, fourteen calendar days for ten working ones, and the count starts on the next working day, which is the usual SLA convention. Public holidays are never deducted automatically here, because they differ by country, by region within a country, and by year.
What people use it for
- Finding a date a set period ahead
- Working out a 90-day payment term
- Scheduling a follow-up a fixed number of days out
- Planning backwards from a deadline
- Finding when a 30-day look-back window opened
- Setting an SLA or delivery deadline in working days
- Working out monthly capacity from the weekday count
- Checking whether a year is a leap year
- Settling an argument about how long February is
- Checking what day of the week a date falls on
Questions
Date addition and subtraction, working days added or counted back, the gap between two dates, and the facts about a single date, month or year.
Type the number and read the result, with its weekday beside it. A hundred days from Thursday 1 January 2026 is Saturday 11 April 2026. Counting back thirty days from a Wednesday lands on a Monday, because thirty days shifts the weekday by two in either direction.
No. A month keeps the day number and clamps short months; 30 days is a fixed count. Thirty days before 31 March is 1 March, and one month before is 28 February.
28 February, or 29 in a leap year: the day clamps rather than spilling into March. One consequence catches people out, which is that the operation does not undo itself. Take a month back off 28 February and you land on 28 January, not on the 31st you started from.
Only when no 29 February falls in between. Otherwise you land a day early.
Two working weeks; fourteen calendar days if no public holidays fall in between.
No. Day one is the first working day after the start, which is the usual SLA convention. Start on a Saturday and the first added day is the Monday.
Not automatically. They vary by country and region, so deduct them yourself.
Twenty-eight, or twenty-nine in a leap year. It is the only month whose length depends on the year, and it is short because the Roman calendar it descends from began with March and left the winter largely unnumbered.
January, March, May, July, August, October and December. Only two pairs of them are adjacent: July with August, and December with January.
Between 20 and 23. A 28-day February always has exactly 20, because 28 days is four complete weeks.
365, or 366 in a leap year: divisible by four, unless divisible by 100, unless also divisible by 400. So 1900 was not and 2000 was. That rule holds the average at 365.2425 days, which drifts from the sun by about a day every 3,200 years.
Between 260 and 262, depending on where the weekends fall. Close to a one per cent spread.
A week numbering where week 1 contains the first Thursday of the year, so early January can fall in the previous year week 52 or 53.
Throughout; including 29 February as a start or end date.