Half birthday calculator: when is my half birthday?

What is a half birthday?

A half birthday is the day you're exactly six months past your last birthday, halfway to your next one. Someone born on March 10 has a half birthday on September 10. Someone born on November 2 has one on May 2. It isn't an official occasion, and nobody expects a present. It's an excuse for a small celebration when a whole year feels like too long to wait.

People mark it for a few different reasons. Parents often celebrate a baby's first half birthday, since six months old is a milestone in its own right. Kids with summer birthdays sometimes get their class celebration on the half birthday instead, because their real one falls during the school holidays. And some people just like having two days a year that are a little bit theirs.

How to work out your half birthday

Count forward six months and keep the same day of the month. That's the whole method. A quick way to do it in your head is to pair the months: January goes with July, February with August, March with September, April with October, May with November, and June with December. Whichever month you were born in, your half birthday is in its partner month, on the same date.

So a birthday on July 19 gives a half birthday on January 19. A birthday on December 25 gives one on June 25. The calculator above does the same thing, then adds the part that's fiddly to do by hand: the day of the week it falls on this time, and how many days you have left.

When the date doesn't exist

The pairing breaks for a few dates at the end of a month. Six months after March 31 is September 31, and September only has 30 days. Six months after August 31 is February 31, which never exists. When that happens, this calculator uses the last day of the month instead. So March 31 becomes September 30, and August 30 and 31 become February 28, or February 29 in a leap year.

The dates that move are March 31, May 31, August 29 to 31, October 31 and December 31. August 29 only moves in ordinary years. In a leap year February has a 29th, so the half birthday stays on it. There's no official rule for any of this. Using the last day of the month keeps the half birthday in the right month. If you'd rather celebrate on the first of the next month, that's a fair call too.

Leap-day birthdays work the other way round. If you were born on February 29, your half birthday is August 29, which exists every year. So a leap-day baby gets a half birthday every single year and a real birthday only one year in four.

Pair the months, keep the date. January with July, March with September, June with December.

How do you celebrate a half birthday?

Small is the point. A half birthday is an excuse, not an event, so the best celebrations are the ones that take ten minutes:

• Half a cake. Bake or buy a cake, cut it in half, and serve one half. It's the classic joke, and it photographs well.
• A photo at six months. For a baby's first half birthday, the monthly photo becomes the halfway photo, often with a "½" sign.
• A school celebration. Kids with summer birthdays often mark the half birthday in class, since their real one falls in the holidays.
• A halfway message. A short text to a friend or partner: "Halfway to your birthday, and I already know what I'm getting you."
• Half-price treats. Split something in two: half a pizza each, half a pint, half a day off.

Is it exactly half a year?

Close, but not to the day. A year is 365 days, or 366 in a leap year, so half of it is 182 and a half days, and months aren't all the same length. Depending on your birthday, six calendar months can be anywhere from 181 to 184 days. Nobody counts the days for a half birthday, though. The same date, six months on, is what everyone means, and it's what this calculator gives you.