How this calculator works
The birthday is rebuilt in the reference year with the same month and day. If that date has already passed, the reference year is increased by one. Comparing calendar day numbers then gives the exact number of days until the birthday. A February 29 birthday is placed on February 28 in a common year so that it still occurs once a year.
Formula
next = birthday in the reference year, or in the following year when its day number is before the reference day number. days until = O(next) - O(reference). age on that birthday = next year - birth year. weekday = 1 + ((O(date) - 1) mod 7), where 1 is Monday.
Worked example
Born 1990-05-20 and checked on 2026-11-01: the next birthday is 2027-05-20, 200 days away, and the person turns 37. The birth date fell on a Sunday and the next birthday falls on a Thursday.
Advertisement
Frequently asked questions
Does my birthday today count as zero days away?
Yes. If the birthday equals the reference date, the next birthday is that same date and the count is zero. The age shown is the age reached on that day.
How is a leap-day birthday handled?
In a common year it is treated as February 28, so the birthday appears once every year. Some people prefer March 1; the day count would then differ by one or two days in a leap year.
Why does the weekday of my birth matter?
It is a common trivia question, and it also verifies the date you entered. The page computes it from the proleptic Gregorian calendar and does not depend on the current date.
Can I use this for an anniversary instead of a birthday?
Yes, the arithmetic is the same. Enter the original date and the reference date. The anniversary calculator adds the number of years together.