Find out how long until your next birthday and what day of the week you were born.
How to Use the Birthday Calculator
Enter a date of birth. The calculator returns your current age, the number of days until your next birthday, the day of the week you were born on, and the day of the week your next birthday falls on.
The site's Age Calculator already gives a precise years/months/days breakdown as its main job, with an optional "as of" date for calculating age at any point in time. This one is a lighter, faster cousin built for a narrower job: how long until the next birthday, and a bit of trivia about the day itself, which the Age Calculator doesn't try to answer.
All of the date math runs on JavaScript's native Date object rather than a hand-built calendar table, which is what correctly handles February, leap years, and month-length differences without extra logic bolted on.
Your Birthday Moves to a Different Weekday Almost Every Year
A year is 365 days, and 365 isn't divisible by 7 — it's 52 weeks plus one extra day. That extra day is why a birthday lands one weekday later each year than it did the year before: born on a Tuesday, and next year's birthday falls on a Wednesday, then a Thursday the year after. A leap year adds a second day to that shift, since February 29 inserts an extra day into the count, so any birthday with a leap day between it and the previous one jumps two weekdays instead of one.
It's a small, checkable fact rather than a coincidence — mark your birthday's weekday for a few years running and the one-then-one-then-two pattern, broken only by leap years, shows up reliably. The day you were born works the same way, just running the pattern backward from a fixed point; a specific calendar date has exactly one weekday, permanently, no ambiguity involved.