Date Calculator
Add or subtract days, weeks, months, years or weekdays from a date, or count the days between two dates, with the day of the week, the day of the year and the ISO week number in the answer. Month arithmetic states its rule: 31 January plus one month is 28 February, and the page says so instead of rolling quietly into March.
Result dateLoading today's date.
Adding a month keeps the day number when it can. 31 January plus one month is 28 February (29 in a leap year), and the line under the result says when that happened instead of rolling quietly into March. Weekday counts drop Saturdays and Sundays only. For public holidays use the business days calculator; for hours and minutes between two moments, the duration calculator.
Common questions
- What happens when I add a month to the 31st?
- The result keeps the same day number when the target month has it, and lands on the last day of the month when it does not. So 31 January plus one month is 28 February (29 in a leap year), 31 January plus two months is 31 March, and 30 November plus three months is 28 February. The line under the result says which of the two happened, so a clamped date is never a silent surprise.
- How are leap years handled?
- By the Gregorian rule: every fourth year is a leap year except century years, unless the century is divisible by 400, so 2000 and 2024 are leap years and 1900 and 2100 are not. Adding one year to 29 February gives 28 February, adding four gives 29 February again, and a span that crosses a 29th of February counts it as a day like any other.
- What is the difference between days and weekdays?
- Days walk straight through the calendar, weekends included. Weekdays are Monday to Friday only: as a unit, they step over Saturdays and Sundays, so Friday plus one weekday is Monday; as a count, they tell you how many Monday-to-Friday days lie in the span. Public holidays are not known here. For a count with US federal or UK bank holidays removed, use the business days calculator.
- Does the count include both dates?
- The day count is the nights between the two dates, so Monday to the next Monday is 7 and 1 September to 30 September is 29. The weekday and weekend-day counts include both dates, the way "working days in September" counts the 1st and the 30th: September 2026 has 22 weekdays and 8 weekend days, 30 in all. The page says which convention each figure uses under the result.
- Do time zones or daylight saving affect the result?
- No. Every date is treated as a calendar date, not a moment on a clock, so a clock change during the span can never add or lose a day. The only place your device's time zone matters is the Today button, which reads the date as your calendar shows it.
- How is the week number worked out?
- By ISO 8601: weeks start on Monday and week 1 is the week that holds 4 January. A week belongs to the year that holds its Thursday, so 1 January 2027 falls in week 53 of 2026. The week number tool shows the US convention alongside if you need that instead.
- Can I count backwards?
- Yes. A negative change subtracts, so -30 days goes back a month, and in Days between mode an end date before the start counts backwards and the page says so. Dates from year 1 to year 9999 are accepted.
Dates are calendar arithmetic on the civil calendar, so daylight saving never shifts a result. Adding a month lands on the same day number when it exists and the last day of the month when it does not, and the page says which happened. Weekday counts exclude Saturdays and Sundays only; public holidays are not known.