How we calculate
All of our tools share a single calculation engine. Rather than working from millisecond timestamps — which can drift across daylight-saving changes and time zones — we calculate directly on the calendar using the year, month and day. This makes day counts immune to time-zone and DST effects.
Years, months and days
To break an age or interval into years, months and days, we subtract the two dates field by field. When the day component would be negative, we borrow one whole month using the length of the earlier date’s month. This is the convention used by official registries and clinical assessments, and it means “one day before your birthday” correctly reads as, for example, 29 years 11 months 30 days rather than 30 years.
Total days, weeks and hours
For totals we convert each date to an absolute day number using a standard integer algorithm (equivalent to the Julian Day Number) and subtract. Every leap day in the range is therefore counted exactly. We cross-checked this against thousands of randomly generated date pairs to confirm it matches an independent reference for every case.
Leap years
A year is a leap year if it is divisible by 4, except for century years, which must be divisible by 400. So 2000 was a leap year but 1900 and 2100 are not. People born on 29 February have their anniversary treated as 1 March in non-leap years, which is the most widely used convention.
Privacy
Every calculation runs entirely in your browser. The dates you enter are never sent to a server.