What it does
Three small date calculators in tabs. Age gives an exact age in years, months and days, the days lived and the next birthday. Days between counts days, weeks, months and weekdays (Monday to Friday) between two dates. Add or subtract moves a date by days, weeks, months or years.
Each tab has an example filled in, and the answer updates as you type. Everything runs in your browser and nothing is sent anywhere. To get a reminder before a passport or permit runs out, use expiry reminders; to turn a date into a Unix time, try timestamp converter.
How to use it
- Pick a tab.
- Choose the dates with the date boxes (and, for adding, the number and unit).
- Read the answer and the sentence under it.
What each setting does
- Date of birth / Age on: the age is worked out on the second date, which starts as today.
- From / To: if To is before From, the two are swapped and the page says so.
- Count the end day too: off, it counts nights (1 January to 2 January is 1 day); on, both days count (2 days).
- Add or subtract, How many, Unit: whole numbers only. Weeks are 7 days.
Worked example
Someone born on 15 May 1990 is 36 years, 5 months, 26 days old on 10 November 2026: 13,328 days. Their next birthday is Saturday 15 May 2027, 186 days later.
From 1 January 2026 to 31 December 2026 is 364 days, or 365 with the end day counted. That span has 260 weekdays, or 261 with the end day, which is a Thursday.
Adding 1 month to 31 January 2026 gives 28 February 2026.
FAQs
What happens when the month is shorter? Adding months keeps the same day of the month, but when that day does not exist the answer moves back to the last day of the month. So 31 January plus 1 month is 28 February (29 February in a leap year), and 31 March minus 1 month is 28 February.
When is the next birthday for someone born on 29 February? In common years the page uses 28 February.
Are public holidays taken off the weekday count? No. They differ by region.
Where the numbers come from
- Gregorian calendar arithmetic; the month-end rule (31 January plus 1 month is 28 February) is the common convention used by spreadsheets and most date libraries. fibub's own working.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Read more on the Blog
Last updated 2026-11-10.