How days alive are counted
Your day count starts at 0 on the day you were born, just as age in years starts at 0. Every calendar day after it adds one, leap days included. 10,000 days old is the date 10,000 days after your birthday.
Counting from day 0
On the day you are born you are 0 days old, and the next day 1 day old. This is the same rule as for age in years: nobody is 1 year old on the day of their birth. So the day you turn 10,000 days old is exactly 10,000 days after your birthday, and 1,000 weeks old is 7,000 days after it.
Calendar days, no time zones
The site counts calendar dates, not hours. Your birthday is a date, today is a date, and the number of days between them does not depend on where you are or on the clock. Leap years add their 29 February like any other day: the Gregorian calendar has a leap year every four years, except century years not divisible by 400, so 2000 was a leap year and 2100 will not be.
Birthdays on 29 February
In years without 29 February, a birthday on that date has to fall somewhere. Anthrotempo uses 28 February by default and lets you choose 1 March instead in the tools. The day count is unaffected: only the birthday dates move.
Only a birth year
With only a birth year, the number of days lived is a range, from a birthday on 31 December to one on 1 January. The tools show that range rather than inventing an exact count, and ask for the full date only for the day and week tools. The date stays in your browser.
Last updated 26 October 2026