How our calculators work
A quick guide to the calendar rules behind the numbers.
Exact age: completed years, then months, then days
We start at the birthday and count full calendar years that have elapsed. We then count full calendar months from that anniversary and the remaining number of days. This is more useful than estimating age by dividing the day count by 365 or 30.
Leap years
The Gregorian calendar adds 29 February when a year is divisible by 4, except century years must also be divisible by 400. For example, 2024 and 2000 were leap years; 1900 and 2100 are not.
29 February birthdays
For birthday countdowns and recurring anniversaries, AgeGlide uses 28 February in non-leap years. This is a presentation convention, not a universal legal rule: official age calculations may follow a different date in your jurisdiction.
End-of-month dates
When adding months, an impossible target day moves to the final valid day of that month. For example, January 31 plus one month lands on February 28 in a common year.
Calendar days versus elapsed time
Date-only tools count calendar-day differences using UTC-based date arithmetic, which avoids errors from daylight-saving time. Date-and-time tools, by contrast, compute elapsed milliseconds from time stamps in your device's local time zone. If an event happened elsewhere, enter its correct time-zone equivalent for precise elapsed hours.
Business days
Weekdays are Monday through Friday, and both start and end dates are counted if they are weekdays. Public holidays are not removed because their dates differ by country and region.
Scientific and entertainment estimates
Pet-age comparisons, generational labels, astrology and Martian-year calculations are approximations or entertainment tools. The calculator pages explain their assumptions.
Useful reference material
Read about leap years, ISO 8601 date formats or our privacy approach.