Why age calculators sometimes give different answers

Enter the same date of birth into two age calculators and you can get two different answers, usually by a day, occasionally by a month or even a year. Neither is necessarily wrong. Age calculation involves a few choices, and calculators don’t all make the same ones. Here are the usual culprits.

What is one month after 31 January?

This is the most common cause of disagreement. Take someone born on 31 January 2023:

The first column is how our calculators work, and how most calendar software adds months: when the target month is too short, the monthly birthday moves to its last day. The second rolls it into the next month instead. The third is the schoolbook subtraction method, and on 1 March it produces a negative number of days, which some calculators display as-is and others “fix” in different ways. If two calculators disagree for someone born late in a month, this is almost certainly why.

Leap day birthdays in common years

People born on 29 February have no birthday on the calendar three years out of four. Some calculators treat 28 February as the birthday in those years, others 1 March, so for a single day each year they disagree by a whole year of age. We cover this in our guide to leap day birthdays. Our calculators default to 28 February and let you switch.

Counting the first day, or not

Age is counted like a distance: on the day you’re born you are 0 days old. Some day counters instead count every calendar day in a range, including the first one, which adds one day. Neither is wrong, but they answer different questions: “how many days old am I?” versus “how many days have I been alive, including the first?” The date difference calculator lets you choose with its “include the end date” option.

Time zones and when “today” changes

A calculator has to decide what today’s date is. Ours uses your device’s time zone, so the date changes at your midnight. A calculator that works in UTC (Greenwich time) changes date at a different moment, so for some hours each day it’s working with yesterday’s or tomorrow’s date. If you check your age late at night or early in the morning, that alone can produce a one-day difference.

Daylight saving can cause trouble too, in tools that count time in milliseconds and divide by 24 hours: the 23-hour and 25-hour days around clock changes can push a result over or under a day boundary. Working with calendar dates, as our calculators do, avoids this completely.

Dividing by 365 or 365.25

Some tools work out age as the number of days divided by 365. That ignores leap days, so the result creeps ahead of the calendar by about a day every four years: by age 80 it’s roughly 20 days ahead. Dividing by 365.25 is closer, but it’s still an average. Near a birthday, either shortcut can show the wrong age in years. Counting calendar years, months and days avoids rounding altogether.

A different way of counting age altogether

Not every system starts at zero. Under traditional Korean age reckoning, a baby was 1 at birth and everyone got a year older on 1 January, so a person could be one or two years “older” than their international age. South Korea moved official documents to international age in June 2023, but traditional counting survives in conversation and some laws still use the calendar-year age. If a calculator gives an age one or two years higher than expected, check which system it uses. Our guide to how age is counted around the world explains these systems.

Old dates and software limits

Some tools can’t handle dates before 1900, and spreadsheets have their own quirks: Microsoft Excel treats 1900 as a leap year for compatibility with older software, which affects dates before 1 March 1900. For dates before the 1700s, check whether the record uses the Julian calendar, which many countries used before switching to the Gregorian one.

How to test any age calculator

Try these dates in any calculator to find out which conventions it uses:

• Born 31 January 2023, age on 28 February 2023. “1 month” means it counts forward and keeps month ends; “28 days” means it rolls over or borrows.
• Born 29 February 2000, age on 28 February 2001. “1 year” means 28 February counts as the birthday; “11 months and 30 days” means it uses 1 March.
• Born today. Anything other than 0 days means it counts inclusively.

Our own choices, and the reasons for them, are all written down in our methodology.