Calculating Age by Hand: Leap Days, Cut-off Dates and Spreadsheets
By Akmal Ali · Published · Updated · 4 min read
Count the complete years since the date of birth, then the complete months since the last birthday, then the remaining days. For someone born on 20 August 1994, on 5 March 2026 that gives 31 years, 6 months and 13 days.
Working out someone’s age in whole years is easy. Getting the exact age in years, months and days is trickier, because months have different lengths and leap years add an extra day, and an application form that sets an age limit on a cut-off date leaves no room for being a day out.
Calculating age by hand
Take a date of birth of 20 August 1994 and a current date of 5 March 2026.
- Years: 2026 − 1994 = 32. But the birthday (20 August) hasn’t happened yet in 2026, so subtract one: 31 years. The last birthday was 20 August 2025.
- Months: count whole months from 20 August 2025. 20 September, 20 October … 20 February 2026 is 6 months. 20 March hasn’t arrived, so it’s 6 months.
- Days: from 20 February to 5 March 2026. February 2026 has 28 days, so that’s 8 days to the end of February plus 5 days: 13 days.
Result: 31 years, 6 months and 13 days.
Age Calculator
Gives the exact age today or on any date you choose, with total days and the next birthday.
The tricky cases
Birthdays on the 29th, 30th or 31st
If someone was born on 31 January, when do they turn one month old? February has no 31st. The common convention, used by our calculator, is the last day of the month: 28 February, or 29 February in a leap year.
Leap-day birthdays
People born on 29 February have a real birthday only in leap years: usually every four years, but not in century years unless they divide by 400, so 2000 was a leap year and 2100 won’t be. In other years, the birthday is usually taken as 28 February. Some legal systems use 1 March instead, so for legal ages, such as the age of majority, check local rules.
Age in a spreadsheet
Excel and Google Sheets both have a DATEDIF function. With the date of birth in A1 and the other date in B1 (or TODAY() in its place):
- Whole years:
=DATEDIF(A1, B1, "Y"), which gives 31 for the example above - Months after the last birthday:
=DATEDIF(A1, B1, "YM"), which gives 6 - Days after the last whole month:
=DATEDIF(A1, B1, "MD"), which gives 13
Microsoft warns that the “MD” unit can give wrong results around month ends, so check those cases against a calculator. For total days, subtract the dates: =B1-A1.
How do you check an age limit on a cut-off date?
Exams, schools, jobs and sports often set an age limit “as of” a fixed date, such as 1 January or 31 August, not today. Calculate the age on that date, and read the wording carefully: “under 25” and “25 or younger” are different limits.
- Under 25 on 1 January 2027. Born 2 January 2002: 24 years, 11 months and 30 days, so yes. Born 1 January 2002: exactly 25 that day, so no. Born 31 December 2001: 25 years and 1 day, so no.
- At least 18 on 31 August 2026. Born 31 August 2008: turns 18 that day, so yes. Born 1 September 2008: 17 years, 11 months and 30 days, so no, by a single day.
To check a case like these, set “Age on this date” in the age calculator to the cut-off date. For a leap-day birthday close to a cut-off, confirm which rule the organisation uses. To count the days between any two dates instead, use the date difference calculator.
Frequently Asked Questions
Why do calculators disagree about an age in months?
They agree on whole months but not on month ends. From 31 January, some count one month on 28 February and others roll over to 3 March. The VektraTool calculator uses the last day of the month.
How many days old am I?
Count the days between the two dates, including every 29 February. Someone born on 20 August 1994 is 11,520 days old on 5 March 2026.
On what date does someone turn a given age?
Add the years to the date of birth: someone born on 20 August 1994 turns 40 on 20 August 2034. For a 29 February birthday in a non-leap year, the organisation’s rule decides between 28 February and 1 March.