How this calculator works
Each date is turned into its day number in the proleptic Gregorian calendar and the birth day number is subtracted from the as-of day number. Every intervening date adds exactly one, whether it is a short February, a 31-day month or February 29. The same count is also expressed as completed years, months and days, which is how age is normally described even though it is a different way of cutting up the same interval.
Formula
age in days = O(as-of date) - O(date of birth), where O(d) is the day number of date d with 0001-01-01 as day 1. The year, month and day breakdown is found by stepping whole calendar years, then whole months, then the remaining days.
Worked example
Born 1990-05-20 and measured on 2026-11-01, you are 13,314 days old. The same span is 36 years, 5 months and 12 days. The first birthday adds 365 days, while the fourth adds 366 because 1992 is a leap year.
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Frequently asked questions
Does the day I was born count as day one?
No. The birth date is day zero and the next calendar date is day one, so the answer is the number of day boundaries crossed. If you want the count that includes the birth date, add one.
Why is the day count bigger than 365 times my age?
Leap years add roughly one day every four years. Over 36 years that is nine leap days, so the exact count is about 13,314 rather than 13,140.
Is the time of birth used?
No. Both inputs are plain calendar dates, so a birth in the morning and one in the evening on the same date give the same answer. A birth time would shift the answer by less than a day and cannot be expressed without a time zone.
Can I measure a pregnancy or a pet age this way?
Yes. Enter the start date and the date you are measuring. The arithmetic does not care whether the dates are a birth, a conception estimate or an adoption day.