Birthday calculator

Type your birth date in whichever way you write it: `12 January 1982`, `1982-01-12` or `01/12/1982`. Everything appears at once, from your exact age and how many days you've been alive to the day of the week you were born, your next birthday and half birthday, your next milestone day, your age on the other planets, and the Moon that day.

For a date written all in numbers, like `03/04/1990`, Date order decides whether the month or the day comes first. If the numbers make it obvious, like `25/12/1990`, the setting doesn't matter.

The date never leaves your device. Today's date comes from your own clock, so the answers are right wherever you are.

The calculator counts the calendar days from your birth date to today, leap days included. Someone born on 12 January 1982 is 16,335 days old on 3 October 2026, which is 2,333 weeks and 4 days.

Hours and seconds are the days multiplied out, 24 hours and 86,400 seconds each, so they count from midnight on the day you were born and can be out by up to a day's worth for your actual birth time. A billion seconds is 11,574 days and a few hours, a little over 31 years and 8 months, and the calculator gives the date you reach it.

Years and months are counted the way people count them: whole years since your last birthday, whole months since the day of the month you were born, then the days left over. Months are uneven, so when your birth date is the 31st and the month is shorter, the count lands on the month's last day.

Leap days make the difference between counting days and counting years. The Gregorian calendar adds 29 February every four years, but skips it in century years unless they divide by 400. That's why 1900 had no 29 February and 2000 did, and why the day count here works through the real calendar instead of multiplying by 365.25.

A year is one trip round the Sun, and every planet takes a different time. The calculator divides your age in days by each planet's orbit in Earth days, using the figures from NASA's Planetary Fact Sheet: 88.0 days for Mercury, 224.7 for Venus, 687.0 for Mars, 4,331 for Jupiter, 10,747 for Saturn, 30,589 for Uranus and 59,800 for Neptune.

At 16,335 days old you'd be 185 on Mercury and 23.8 on Mars, and not yet 1 on Saturn, which takes almost 29.5 Earth years to go round once.