Everyday guide
How to Calculate Age and the Number of Days Between Two Dates
Updated 28 September 2026 · 3 min read
Date maths looks easy until months of different lengths, leap years and "do I count the last day?" get involved. Here's how to do it by hand, and when to hand it to a calculator. For an instant answer, use the Age Calculator or the Days Between Dates calculator.
Open the free Age CalculatorHow to calculate someone's exact age
- Years: subtract the birth year from the current year, then take off one if the birthday hasn't happened yet this year.
- Months: count the whole months since the last birthday.
- Days: count the days since that month-anniversary.
Example: born 14 March 1990, today 28 September 2026. The 2026 birthday has passed, so 2026 − 1990 = 36 years. From 14 March to 14 September is 6 months, and 14 to 28 September is 14 days — 36 years, 6 months and 14 days.
Why two calculators can disagree by a day or a month
Months aren't all the same length, so "one month after 31 January" has no perfect answer. Most calculators, including ours, treat a birthday or start date late in the month as reaching its month-anniversary on the last day of a shorter month. Totals in days are never ambiguous — if you need an exact figure for a contract or a visa, work in days.
How to count the days between two dates
The number of days between two dates is the difference between them on the calendar. By hand, count the days left in the first month, add the full months in between, then add the days into the final month — remembering February has 29 days in a leap year.
Example: 1 March to 15 April. March has 31 days, so there are 30 days from 1 to 31 March, plus 15 days into April: 45 days.
Should you include the end date?
This is the most common source of "off by one" answers. Counting from one date to another usually leaves out one end — like counting nights in a hotel: check in on the 1st, out on the 4th, three nights. When you need both days counted — a leave request from the 1st to the 4th inclusive is four days — add one. The Days Between Dates calculator has an "include the end date" option for exactly this.
Working days, not calendar days
Deadlines, notice periods and delivery times are often in business days. The Working Days Calculator skips weekends — including Friday–Saturday weekends — and any public holidays you list. Public holidays aren't added automatically, because they differ by country and even by state.
Adding days to a date
To find the date 90 days from today, or 6 weeks before a deadline, use the Date Calculator. It handles month lengths and leap years for you: 31 January plus one month gives the last day of February, not a date that doesn't exist.