About this tool
The Birthday Calculator takes a birth date and answers the questions people actually ask about it: what day of the week it was, how old you are today, when the next birthday falls and on which weekday, how many days are left until then, and when the half-birthday lands. Below that it lists every milestone birthday from 18 to 100 with the calendar date and weekday of each, plus the less obvious 10,000-day and 20,000-day birthdays. It is useful for party planning, for checking which milestone birthdays land on a weekend, and for settling the question of what day someone was born.
How it works
The birth date and the reference date are parsed as local calendar dates at midnight. The weekday comes straight from the calendar. Age in whole years is the number of birthday anniversaries on or before the reference date, where a February 29 anniversary falls on February 28 in non-leap years. The next birthday is the first anniversary strictly after the reference date, and the countdown is Math.round((next - today) / 86,400,000) so daylight-saving changes never shift it by a day. Milestone dates are the birth date with the year advanced by 18, 21, 30 and so on, with the same leap-day clamp. Day-count birthdays add exactly 10,000 or 20,000 days to the birth date, and the half-birthday is the birth date plus six months, clamped to the month's last day.
Frequently asked questions
What is a 10,000-day birthday?
It is the day exactly 10,000 days after you were born, which falls at about 27 years and 4.5 months of age. Someone born on January 1, 2000 reaches it on May 19, 2027. The 20,000-day birthday comes at roughly 54 years and 9 months. They are a popular alternative to round-number birthdays because they land on unexpected dates.
When is my half-birthday?
Six months after your birthday. June 15 gives December 15, and October 20 gives April 20 of the following year. If the day does not exist six months on, for example August 31, the calculator uses the last day of that month, so February 28 or 29. Half-birthdays are often used by families with a child born near a major holiday.
Why does the calculator show my next birthday on February 28 when I was born on February 29?
February 29 only exists in leap years, so in the other three years there is no exact anniversary. This calculator uses February 28 as the anniversary in non-leap years, which is the most common convention; some countries and some organizations use March 1 instead. In a leap year the anniversary is February 29 as normal.
How does it know the weekday of a date hundreds of years ago?
It uses the browser's built-in calendar, which applies the proleptic Gregorian calendar to every date. That gives the right weekday for any date after the Gregorian calendar was adopted where you live (1582 in most Catholic countries, 1752 in Britain and its colonies). For older dates the historical Julian weekday may differ.