Birthday Calculator

How it works

The calculator reads the day, month and year from your date of birth. The weekday you were born comes straight from the calendar, because every date maps to exactly one day of the week. Your age is the number of completed years since your birth: the current year minus your birth year, less one if this year's birthday has not happened yet. For the countdown, the calculator finds the next occurrence of your birthday: this year's date if it is still ahead, otherwise the same date next year, then counts the whole days between today and that date. Today itself is not counted but the birthday is, so the result falls by one each midnight until it reaches 0 days on the birthday itself. For 29 February birthdays the next occurrence is taken as 1 March whenever the year is not a leap year, which keeps both the age and the countdown consistent from year to year.

Worked example

Take someone born on 15 January 1990. The calendar puts that date on a Monday, so that is the day they were born. Their age is the completed years since 1990: once the 15 January birthday has passed in a given year, they have turned the current year minus 1990. The countdown works the same way for any date: if your next birthday falls on 15 January and today is 15 December, there are 31 whole days between the two dates, so the calculator shows 31 days. On 14 January it shows 1 day, and on 15 January it shows 0 days along with the weekday the birthday lands on, which shifts forward one weekday most years and two after a leap day.

Related calculators

• Age Calculator: your exact age in years, months and days.
• Birth Year Calculator: work out a birth year from an age.
• Countdown to Date Calculator: days until any date you choose.
• Date Calculator: add or subtract days between dates.