Date arithmetic looks trivial and is not, because months are different lengths, years are not all the same length, and the word "between" is ambiguous.
How do you work out your exact age?
Work down the units in order, largest first. Subtract the years, then the months, then the days, borrowing where you have to.
The working
Born 14 March 1990, today 16 August 2026 Years: 1990 → 2026, and 14 March has passed = 36 years Months: 14 March 2026 → 14 August 2026 = 5 months Days: 14 August → 16 August = 2 days → 36 years, 5 months, 2 days
The order matters. Trying to do it in one step — or converting everything to days and dividing by 365.25 — gives an answer that is close but wrong, and visibly wrong near a birthday.
The inclusive-or-exclusive question
How many days from Monday to Wednesday? Two if you count the gaps, three if you count the days themselves. Both are correct answers to different questions, and neither is wrong — but you have to know which one you need.
- Counting the gap — exclusive — is what you want for durations, notice periods and ages. Monday to Wednesday is 2 days.
- Counting the days — inclusive — is what you want for booking nights, holiday entitlement and anything you are being charged per day for. Monday to Wednesday is 3 days.
Leap years
A year is a leap year if it divides by 4, except century years, which must divide by 400. So 2024 was, 1900 was not, and 2000 was.
This is why 365.25 is only an approximation, and why anyone born on 29 February has a legitimate grievance. In the UK, for legal purposes such a birthday falls on 1 March in non-leap years.
Common questions
How many days are in a month, on average?
30.44, which is 365.25 ÷ 12. It is a useful figure for rough planning and a bad one for anything exact — no actual month is 30.44 days long, so any calculation that relies on it will drift.
Why does my age come out differently on different sites?
Almost always the inclusive-or-exclusive question, or a timezone. A site working in UTC while you are in British Summer Time can be a day out for part of the year. If it matters, check what the tool counts.
How do I count working days?
Count the total days, subtract the weekends, then subtract any bank holidays that fall on a weekday. The bank holidays differ between England and Wales, Scotland and Northern Ireland, which is the part people forget.
How many weeks in a year?
52 weeks and 1 day in a normal year, 52 weeks and 2 days in a leap year. That leftover day is why a given date shifts forward one weekday each year, and two after a leap year.
Check this before you rely on it. A free guide to everyday arithmetic — not tax, accounting, financial, legal or medical advice. The working is shown above so you can verify it against your own figures. See our terms of use.