Decision tools built for clarity
Age Calculator
Calculate completed age and total elapsed days between two dates.
Enter the required values, select the applicable options, and calculate.
Methodology
Formula or transformation
Computes completed Gregorian calendar components and the exact elapsed whole-day interval.
Variables
- Birth date
- Age calculation date
- Completed years
- Completed months
- Completed days
- Total elapsed days
Units
- Year
- Month
- Day
Rounding policy
Calendar components and total days are whole integers; no fractional date is displayed.
Calendar components are calculated as completed Gregorian years, months, and days; total days is the signed date interval.
Calculate completed age and total elapsed days between two dates.
Reviewed sources
- PHP Documentation — DateTimeInterface::diff calendar difference Reviewed on 2026-09-07
Formula review history
-
Initial formula and source review.
Reviewed 2026-09-07 for formula version gregorian-completed-age-v1
Assumptions
- Inputs are interpreted using the displayed units and options.
- The calculation follows the methodology and factors listed in the sources.
Limitations
- Display rounding can hide additional decimal places.
- The calculation cannot add accuracy that is absent from the submitted inputs.
Worked example
A birth date of 7 September 1990 and an as-of date of 7 September 2026 gives 36 years and 13,149 total days.
Frequently asked questions
How are the years, months and days counted?
Completed calendar years are counted first, then completed months, then the remaining days, moving forward from the birth date; total days is the exact number of days between the two dates. A month counts as completed when the same day number is reached, and a day the month lacks (such as 31 February) rolls into the next month: from 31 January to 1 March is 0 months and 29 days, the first month from 31 January is completed on 3 March in a common year, and a birthday on 29 February is completed on 1 March in common years.
Which date is the age measured to?
The as-of date you enter. Both dates must fall between 1900 and 2100, and the birth date cannot be after the as-of date.