The formula
How it works
Find an exact age in years, months and days from a date of birth. Leave the second date blank to use today, or set it to work out how old someone was — or will be — on any particular day.
FAQ
Does it count leap years correctly?
Yes. The calculator works with real calendar dates, so leap days and the different lengths of months are all handled — the months and days it reports always match a calendar.
Can I find an age on a past or future date?
Yes. Set the second date to any day you like — a past anniversary or a future birthday — and the calculator gives the age as of that date instead of today.
Why do the months and days sometimes seem to “borrow” from each other?
When the day of the end date is earlier in the month than the birth day, a full month is borrowed and converted into days so the day count stays positive — the same way you’d borrow when subtracting by hand. It keeps the years, months and days all matching a real calendar.
Can I use this for a pet, a document or an event instead of a person?
Yes — the calculation only needs a start date and an optional end date, so it works equally well for a pet’s age, how long a document has been in force, or the time since any milestone.
Does the time of day or time zone affect the result?
No. The calculator works with whole calendar dates only, so it doesn’t matter what time zone you’re in or what time of day it is — only the dates themselves matter.
How is age in months or weeks useful?
Many milestones — vaccination schedules, developmental checkups, warranty periods — are tracked in months or weeks rather than years, so the total figures let you match an age to those specific windows.
Why might my age in years differ from what a form calculates?
Most discrepancies come from rounding — some systems just subtract birth year from current year without checking whether the birthday has occurred yet this year. This calculator always checks the full month and day, so it never rounds up early.
About the age calculator
This calculator works out an exact age from a date of birth, in years, months and days. It sounds simple, but doing it in your head is surprisingly error-prone because months have different lengths and leap years add a day. By working directly with calendar dates, the calculator gives a precise breakdown, and can also total the age in months, weeks or days for forms and official purposes.
How to use it
Enter the date of birth using the date picker. By default the age is measured as of today, but you can enter a second date to find the age on any other day — useful for working out how old someone will be at a future event, or was at a past one. The result shows the full years, the leftover months and days, and the totals in months, weeks and days. Everything updates the moment you change a date.
The formula
The age is simply the gap between two dates, , but expressed in a calendar-friendly way. The calculator subtracts the years, then the months, then the days, borrowing from the next unit up whenever a figure would go negative — exactly how you would work out a duration by counting on a calendar. The total days come from the raw difference between the two dates in milliseconds.
Where it is used
Age comes up constantly: on official forms, for age limits and eligibility, in medicine where doses and milestones depend on it, and for working out anniversaries and countdowns. Parents use it to track a baby’s age in weeks and months, and event planners use the future-date option to know how old someone will be on the big day. It is one of those small calculations everyone needs from time to time.