When's my half birthday?

The rule is simple

Add six months to your birthday. If that day doesn't exist, use the last day of the month.

March 14 becomes September 14. November 2 becomes May 2. Done! It's the same rule spreadsheets use when they add months to a date (Excel's EDATE works exactly like this), so your calendar, your calculator and your spreadsheet will all agree.

There's a second way to count it. Some people (Wikipedia among them) take exactly half a year instead: 182.5 days in a normal year, 183 when a February 29 is in the way. That lands on the true middle of your year, usually a day or so away from the calendar date. We show it too, in the box under your result. The big date is the calendar one because that's what Google, U.S. retirement rules and every other half birthday calculator we tested go with.

Seven birthdays that break it

The rule only trips when six months later lands in a shorter month. Here's the complete list:

Born on February 29? Lucky you. Your half birthday is August 29, and unlike your actual birthday, it shows up every single year.

Six months isn't exactly halfway

Here's a fun one. A year has 365 days, so the true middle of your year is 182.5 days after your birthday. Six calendar months can be anything from 181 to 184 days, which means your half birthday is always a little early or a little late.

January, February, September and November babies get theirs a day and a half early. March, May, July and August babies wait a day and a half longer. The calculator shows that exact midpoint in its own box, in case you'd rather celebrate the precise middle of your year. Sometimes it's at noon.

The most popular half birthday is February 28

We ran the rule on 85.7 million U.S. births. A normal date gets about 11,200 half birthdays a year. February 28 gets four times that, because it scoops up everyone born on August 28, 29, 30 and 31.

The quietest is June 25, the half birthday of Christmas babies. Then come July 1 (New Year's Day babies), June 24 (Christmas Eve) and January 4 (the Fourth of July). Hospitals schedule far fewer deliveries on holidays, and the dip follows those kids all the way to their half birthday.

Who celebrates half birthdays?

Summer-birthday kids, mostly. If your birthday falls in July or August, it's during school vacation, so lots of classrooms celebrate on the half birthday instead. Cupcakes in January: everybody wins.

Babies, too. Six months is the first big milestone, and a half cake with one candle is practically a tradition by now. Plenty of grown-ups also use it as a mid-year excuse for dessert. We fully support this.

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