Date Calculator
Days between two dates, a date plus or minus days, an age, a week number.
Runs entirely in your browser
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Four questions about the calendar, answered as you change the dates. Days between counts from one date to another, with the end day included if you want it, and breaks the span into weeks, into years, months and days, and into working days and weekend days. Add or subtract moves a date on or back by days, working days, weeks, months or years. Age says exactly how old someone is, in years, months and days, and how long until their next birthday. Week number gives the ISO week of any date, Monday to Sunday, and the US count beside it.
Months are counted the way people count them: one month after 31 January is the end of February, not 3 March, and someone born on 29 February has their birthday on the 28th in other years. Leap years are in. Nothing is sent anywhere; the arithmetic runs in this tab.
How to use it
- Pick what to work out: days between, add or subtract, age, or week number.
- Choose the dates. Today is a tap away.
- Read the answer, and the breakdown beneath it.
Questions
Does it count the start and end day?
It counts the days from the start to the end, the way a calendar subtraction does: 1 to 5 January is 4 days. Tick Count the end date as well for 5, which is what a booking from the 1st to the 5th inclusive means.
What counts as a working day?
Monday to Friday. Public holidays are not taken out, because they depend on the country, the region and the year, and a guess would make the number look more exact than it is.
What is the ISO week number?
The week numbering in ISO 8601, used across Europe and by most software: weeks run Monday to Sunday, and week 1 is the one with the year's first Thursday in it. So 1 January can fall in week 52 or 53 of the year before. The US count, with weeks from Sunday and week 1 holding 1 January, is shown too.
How is a month added to the 31st?
It lands on the last day of the next month when that month is shorter: 31 January plus one month is 28 February, or the 29th in a leap year. Adding a year to 29 February gives 28 February.
One question each
The same calculator, opened on the question you came with:
- How Many Days Between Two Dates
- Add or Subtract Days from a Date
- Age Calculator: How Old Am I
- What Week Is It
Your data stays on your device
Everything above runs inside your browser as WebAssembly compiled from Rust. Nothing you type is uploaded, logged or stored on a server. You can load this page once, go offline, and it still works.
This page makes no requests at all, to anywhere. That is not a promise in the copy: it is a Content-Security-Policy header your browser enforces, and connect-src on it is none. Open the network tab and watch nothing happen.