Calculate Birthday Date
Know someone's exact age but not their date of birth? Enter the age in years, months and days, and the date the age was measured on — the calculator works backwards to the birth date and the day of the week.
- Birth date
- Friday, August 7, 1992
- ISO date
- 1992-08-07
- Day of the week born
- Friday
Worked example
Example: a patient is 34 years, 2 months and 10 days old on October 17, 2026
- Go back 34 years: October 17, 1992
- Go back 2 months: August 17, 1992
- Go back 10 days: August 7, 1992
Birth date: Friday, August 7, 1992.
Formula
Birth date = Reference date − years − months − days
Years and months are subtracted on the calendar first (clamping to the end of shorter months), then the days.
Frequently asked questions
What year was I born if I am 16?
Subtract 16 from the current year. If you've already had your birthday this year, that's your birth year; if not, subtract one more. In 2026: born in 2010 (birthday passed) or 2009.
How do I find what day of the week I was born?
Enter your age with years, months and days, or use any date of birth — the result shows the weekday.
Is the result exact?
Yes, when you provide the full age (years, months and days) and the exact measurement date.
Last reviewed October 17, 2026 · Calculations run in your browser; nothing you enter is sent to a server.