How is exact age calculated?
Age is the calendar difference between birth date and the “as of” date, shown as years, months, and days.
Age Calculator
Enter a birth date (and an optional “as of” date) to see exact age broken into years, months, and days.
Calculate exact age
Use today’s date as the default “as of” date, or pick another day for a historical or future age.
About this calculator
How to use Age Calculator, plus the formulas and logic behind the results.
Tool description
The Age Calculator converts a birth date into an exact age on a chosen day. It is useful for forms, eligibility checks, milestones, and answering “how old on this date?” questions.
How to use
- Enter the date of birth.
- Optionally change the “as of” date (defaults to today).
- Select Calculate to see years, months, days, and total days.
Formulas & reference
age = as_of_date − birth_date (calendar Y/M/D)total_days = number of whole days between the two dates
Age outputs
- Years, months, and remaining days
- Total days lived (inclusive calendar span)
- Works for past or future “as of” dates after birth
Calculation logic
The tool uses DateTimeImmutable::diff to compute calendar years, months, and leftover days. Total days come from the same interval’s day count so leap days are included automatically.
Age uses calendar difference (Y/M/D), not a fixed 365.25-day year average.
Age Calculator FAQ
Common questions about this calculator and how to use the results.
Can I find age on a past date?
Yes. Set the “as of” date to that past day.
What if the as-of date is before birth?
The calculator returns an error—age requires the as-of date to be on or after the birth date.
Does it count the birthday as a full year?
On the birthday, months and days reset to 0 for that year boundary, matching standard calendar age.
Are leap-day birthdays handled?
Yes. Diff uses calendar rules; February 29 births advance normally in non-leap years.
Is total days the same as years × 365?
No. Total days counts actual calendar days, including leap days.
Can I use this for pets or companies?
Yes—any start date works as a “birth” or founding date.
Do time zones affect age?
Only calendar dates are used, so time of day and timezones are ignored.