Age Calculator
Exact calendar age from any date of birth — years, months, days, total days lived, and the next-birthday countdown.
"How old" sounds simple until you need it precisely: immigration forms and insurance applications ask for age in completed years, pediatric checkups think in months, and trivia demands the total number of days. This calculator takes a date of birth and gives the exact calendar age — years, months and days — along with total months, weeks and days lived, the weekday of birth, and how many days remain until the next birthday.
The "as of" date makes it more than a birthday toy: set it to a form's deadline to know the age on that exact day, to a historical date for genealogy work, or to a future date to answer questions like "how old will she be when school starts?". The month arithmetic follows standard calendar rules (borrowing real month lengths, handling leap years correctly), so results match how official documents count age. For the gap between two arbitrary dates that aren't birthdays, the date calculator is the general-purpose version.
How to use the Age Calculator
- Pick the date of birth.
- Leave "As of" set to today — or change it to any past or future date.
- Read the exact age, the totals in months, weeks and days, and the countdown to the next birthday.
- The note below the results shows the weekday of birth and which weekday the next birthday lands on.
Frequently asked questions
How is age in years, months and days calculated?
By calendar arithmetic: count full years first, then full months, then leftover days — borrowing from the previous month's real length when the day-of-month hasn't arrived yet. This matches how ages are counted on official forms, not a simple division by 365.
How do leap-year birthdays (February 29) work?
The date math handles leap years exactly — total days are always correct. For the next-birthday countdown in a non-leap year, JavaScript's calendar rolls Feb 29 to March 1, which mirrors how most jurisdictions treat leap-day birthdays.
Can I calculate age at a date in the past or future?
Yes — that's what the "As of" field is for. Set it to a document date, an application deadline, or any future date to see the age on that exact day.
Why does total days not equal years × 365?
Because leap years add a day roughly every four years. Someone exactly 40 years old has lived about 14,610 days, not 14,600 — the calculator counts real calendar days between the two dates.
Is the age calculated in my time zone?
Dates are interpreted as plain calendar dates on your device, with no time-of-day or time-zone component — the same way ages work on paperwork. Someone born "June 5" turns a year older on June 5 wherever they are.