How it works

Bonus birthdays are real, calculated moments — not guesses. Here is exactly how we work them out, and why the maths matters.

Days and weeks: calendar maths

Day and week milestones are simple in idea but precise in practice: your 10,000th day is exactly 10,000 calendar days after the day you were born, and your 1,000th week is 7,000 calendar days after. We count real calendar days — including leap years — and never convert your age to days using 365 or 365.25, because that would drift the date over a lifetime.

Hours, minutes and seconds: elapsed time

These milestones are elapsed durations from the instant you were born, so they depend on your birth time. If you provide it (along with your birth timezone), we calculate the exact instant. If you do not, we assume midnight on your date of birth in your timezone and clearly label those dates as estimates — we never show a falsely precise ticking countdown from a guessed birth time.

Timezones and daylight saving

Elapsed-time milestones are calculated in absolute time, so they are correct regardless of where you are now. We display each date in your birth timezone, and daylight-saving transitions are handled automatically by the timezone engine in your browser.

Consolidating milestones

Some milestones fall on the same instant — three billion seconds is exactly fifty million minutes, for example. We combine those into a single event with both labels, so you never see the same celebration twice.

Privacy by design

Every calculation happens in your browser. Your date of birth and name are not sent to a server, stored in a database, written into the page address, or included in analytics. Read more in our privacy page.