Birthday Calculator
Find your next birthday, how many days away it is, and how old you will turn.
Last updated · View algorithm, constants and default inputs (JSON)
Results
About this calculator
Your next birthday is not simply the same date this year — if it has already passed, it belongs to next year. This calculator works that out and also tells you the weekday it falls on. February 29 birthdays are observed on March 1 in non-leap years.
Worked example
calculated with the default values below
Date of birth 1990-06-15 Days until your next birthday 227 days · Age you will turn 37 years · Next birthday date 2027-06-15 · Day of the week Tuesday · Birthdays celebrated so far 36
General background references
Disclaimer
These calculators give general date results. Public holidays, working-day conventions and time-zone rules are set locally and change, so confirm against the official calendar for anything legally or contractually binding.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
d = (t_next − t_today) ÷ 86 400 000 ms; age = year(t_next) − y_b
- t_next
- Next birthday on/after today
- t_today
- Today at UTC midnight
- d
- Days until birthday
- age
- Age reached on that birthday
- y_b
- Birth year
All dates use UTC midnight; a February 29 birthday falls back to March 1 in non-leap years.
Frequently asked questions
Why might my next birthday show an age one year higher than today?
The tool finds the next occurrence of your birth date from today, rolling into next year once this year's date has passed, then reports the age you will turn on that day. Someone born June 15 who checks in July therefore sees "turns 37" while their last birthday was at 36. Birthdays celebrated so far counts every past annual date, so it is one less than the age you are about to reach.
How are February 29 birthdays handled?
February 29 birthdays are treated as March 1 in non-leap years. The calculator requires a valid date of birth that is not in the future.