Date calculator

Add or subtract days, weeks, months and years from a date, or subtract one date from another to get the gap. Every answer prints its working on a tape, so you can see exactly how month ends and leap years were handled.

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Ask it in plain English

An AI model only translates your sentence into an operation; the result comes from the same tested date engine as the calculators, so the maths is exact.

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What "date maths" actually involves

Counting days is easy until a month runs out. Calendar arithmetic has three awkward units: months of 28 to 31 days, years of 365 or 366, and (once clocks are involved) days of 23 or 25 hours when daylight saving time changes. DateMath Pro works on plain calendar dates as whole-day numbers, so the result never shifts because of your time zone, and it applies the same month-end rule as the ISO 8601 date libraries used by banks and airlines.

Adding days vs adding months

Days are exact: 90 days is always 90 calendar days. Months are not: "3 months from 30 November" is 28 or 29 February, because there is no 30 February. If a contract or notice period says days, add days; if it says months, add months and expect clamping at month ends. The add days to a date page walks through that rule step by step.

Choosing the right calculator

Questions people ask

How does the date calculator add months to 31 January?
It moves to the same day number in the target month and, if that day does not exist, clamps to the last day of the month. 31 January + 1 month is 28 February (29 February in a leap year). This is the rule used by ISO 8601 libraries such as JavaScript Temporal, Java java.time and Python dateutil.
In what order are years, months, weeks and days added?
Years and months first, as one step, then weeks and days. Adding 1 month and 1 day to 31 January therefore gives 1 March (31 Jan → 28 Feb → 1 Mar), not 29 February. The working tape shows each step so you can check it.
Does the calculator count the start date?
Adding 1 day to Monday gives Tuesday: the start date is day zero. For "days between" you can switch on inclusive counting to count both the first and last day.
Can I use it for dates before 1582 or far in the future?
Yes, from year 1 to 9999, using the proleptic Gregorian calendar (the Gregorian rules extended backwards). Historical dates recorded in the Julian calendar need converting first.
How do I subtract one date from another?
Switch to "Between two dates" and enter both. The result is the later date minus the earlier one in days, plus the same gap in weeks, months and years. In a spreadsheet the equivalent is =B1-A1.
How many days are in 6 months or a year?
A year has 365 days, or 366 in a leap year. Six months is 181 to 184 days depending on which months they are. To turn a number of days into years, weeks or months, use the days to years calculator.
How do I add weeks to a date?
Enter the number of weeks: each week is exactly 7 days, so 6 weeks from 1 October is 12 November. Weeks never need month-end rules.
Is my data sent anywhere?
The calculators run entirely in your browser. Only the optional plain-English box sends your sentence to an AI model to be read; the date maths itself still happens on your device.

Other date calculators

Days between datesExact gap in days, weeks and months.Business daysWorking days with US or UK holidays.Time durationHours and minutes across time zones.Days from todayThe date N days ahead or back.Days untilHow many days are left until a date.AgeExact age in years, months and days.Week numberISO 8601 week of any date.Day of year & JulianOrdinal day, YYDDD and Julian Day.Day of the weekWeekday of any date, with the method.Recurring datesEvery N days, weeks or months.Add days to a dateAdd days, with month-end rules shown.Add months to a dateAdd months: EDATE vs spill-over rules.Days to yearsConvert days, weeks, months, years.