Age in days is the number of calendar-day boundaries after a birth date.

Method

The calculator accepts a literal valid YYYY-MM-DD date, turns its year/month/day parts into a calendar day, and subtracts the birth-day serial from the reference-day serial. The birth date is day zero. A reference date one calendar day later therefore produces one day lived.

Worked example

For a birth date of February 28, 2024 and a reference date of March 1, 2024, the result is two days. February 29 is a real calendar day in 2024, so it is counted. Multiplying an age by 365 would incorrectly return one day for this example.

Inclusive counting is a different question

The main age result counts boundaries, so it excludes the starting boundary. A form that says to count both listed dates needs an inclusive date-span calculation instead; adding one is appropriate only when both endpoints are meant to count.

Edge cases and limits

Invalid dates are rejected instead of repaired. The calculator compares local dates, which makes daylight-saving changes irrelevant to the day total. It rejects a birth date after the selected reference date. It does not decide a legal birthday observance, eligibility rule, or time-of-day age.

Use the age on a specific date calculator for a result, the leap-year guide for century rules, and the methodology for the complete contract. The Gregorian calendar rule is described by the timeanddate leap-year reference.