Date calculator

How to use it

1. Pick the tab. Use “Difference” to compare two dates, “Add or subtract” to move a date, and “Date details” for the week number and weekday.
2. Enter the dates. Choose each date with the date picker or type it. Dates between 1900 and 2200 are supported.
3. Set the options. For a difference, decide whether the last day counts. For a shift, enter any mix of years, months, weeks and days, and choose add or subtract.
4. Read and copy the result. The answer updates as you type. Use “Copy result” to paste it into an email or a document.

How the difference between two dates is worked out

The calculator gives the difference in two ways, because people ask the question in two ways. The first is the total number of days, which is a plain subtraction of two calendar days. The second is the calendar duration in years, months and days, the way you would say someone's age or the length of a contract.

Total days never depends on where you are. A calendar date has no time of day, so the clock change in spring or autumn cannot add or remove a day. From 28 March to 30 March 2026 is two days in London, Rome and New York alike, even though one of those nights is only 23 hours long in Europe.

For years, months and days the rule is: take the largest whole number of months that fits between the two dates, then count the days that are left. When a month is shorter than the day of the month you started on, the calculation stops at the end of that month. So 31 January to 1 March 2026 is 1 month and 1 day: 31 January plus one month lands on 28 February, and one more day reaches 1 March.

By default the first day is counted and the last is not, which is how most people count nights of a stay. Switch on “Include the last day” to count both ends, as you would for the days of a course or a holiday booking.

Adding and subtracting years, months, weeks and days

To move a date, the calculator applies years and months first and weeks and days after. The order matters because months have different lengths. Adding one month to 31 January gives 28 February in 2026 and 29 February in 2024, the last day of the shorter month, rather than jumping into March.

A leap day behaves the same way: 29 February 2024 plus one year is 28 February 2025, and plus four years is 29 February 2028. Adding 365 days is not the same as adding a year, because a leap year has 366 days; the calculator lets you choose which one you mean.

• Deadlines: “30 days from the invoice date” is a count of calendar days, while “one month from the invoice date” follows the month-end rule above.
• Subscriptions and trials: add months, not days, so the renewal falls on the same day of the month.
• Planning: add weeks for sprints, courses and milestones that run in whole weeks.

If you need to skip weekends and holidays, use the business days calculator instead.

Week numbers and the ISO 8601 rule

Most of Europe, and many companies worldwide, number weeks by the ISO 8601 standard. A week starts on Monday, and week 1 is the week that contains the first Thursday of the year, which is the same as the week containing 4 January. A consequence is that the last days of December can belong to week 1 of the next year, and the first days of January to week 52 or 53 of the previous year.

A year has 53 ISO weeks when 1 January falls on a Thursday, or on a Wednesday in a leap year. 2026 is such a year: it starts on a Thursday, so December 31, 2026 is in week 53 of 2026. And January 1, 2027, although it is in 2027 on the calendar, is still in week 53 of 2026.

The United States and a few other countries number weeks differently: weeks start on Sunday and week 1 is the one containing 1 January. The two numbers often differ by one, and in some years for the whole year, depending on the weekday of 1 January, so check which system a form or a payroll calendar uses.

Worked examples

Frequently asked questions