A half birthday is the date exactly six months after (or before) your birthday. If you were born on March 10, your half birthday is September 10. Beyond half birthdays, there are plenty of other milestones you can celebrate, such as turning 10,000 days old (at about 27 years and 4 months) or 1 billion seconds old (at about 31 years and 8 months).
How to find your half birthday
Add six months to your birthday, keeping the same day number:
| Birthday month | Half birthday month |
|---|---|
| January | July |
| February | August |
| March | September |
| April | October |
| May | November |
| June | December |
| July | January |
| August | February |
| September | March |
| October | April |
| November | May |
| December | June |
Tricky dates
- August 29, 30 or 31: February has no such day (except the 29th in leap years). Most people celebrate on the last day of February, either February 28 or 29.
- March 31, May 31, October 31 and December 31: the half birthday month has only 30 days, so use the 30th (or the 1st of the next month, if you prefer).
- February 29 (leap day babies): the half birthday is August 29, which exists every year, so it's the one celebration that never moves! See how leap years work.
If you want a mathematically exact midpoint instead, it's 182 or 183 days after your birthday, which can be a day or two off from the calendar method.
Why celebrate a half birthday?
- Summer babies at school: children with birthdays in July or August often never celebrate in class, so many teachers mark half birthdays during the school year instead.
- Holiday birthdays: people born near Christmas or New Year often celebrate their half birthday in June or July, away from the holiday rush.
- Toddlers: "2½" matters a lot when you're two! Parents often track months for babies and halves for preschoolers.
Big-number milestones
| Milestone | Approximate age |
|---|---|
| 1,000 days old | 2 years, 9 months |
| 500 weeks old | 9 years, 7 months |
| 5,000 days old | 13 years, 8 months |
| 1,000 weeks old | 19 years, 2 months |
| 10,000 days old | 27 years, 4 months |
| 1 billion seconds old | 31 years, 8 months |
| 500 months old | 41 years, 8 months |
| 20,000 days old | 54 years, 9 months |
| 500,000 hours old | 57 years |
| 30,000 days old | 82 years, 1 month |
| 1,000 months old | 83 years, 4 months |
The exact date depends on how many leap days you've lived through, so use a calculator for the real day. Enter your birthday in the age calculator to see your age in days, weeks, hours and more.
How to calculate a milestone date
- Choose the milestone, for example 10,000 days.
- Add that many days to your date of birth. In the date duration calculator, you can check the gap between your birthday and any candidate date.
- For seconds or hours, convert first: 1 billion seconds ÷ 86,400 = 11,574 days, plus about 1 hour 46 minutes.
Example: someone born on January 1, 2000 turns 10,000 days old on May 19, 2027, and 1 billion seconds old on September 9, 2031 (counting from midnight on their birthday).
Other dates worth marking
- Golden birthday: turning the age that matches your birth date, such as 15 on the 15th.
- Unbirthday: any of the other 364 days, made famous by Through the Looking-Glass.
- Anniversary milestones: 100 days together, 1,000 days, or a 6-month "half-iversary".
- Sobriety or habit streaks: 30, 90 and 365 days are common markers. Days since keeps count.
- Life in weeks: see your whole life as a grid of about 4,000 weeks with the life in weeks chart.
Count down to the celebration: make a shareable timer for a half birthday or 10,000-day party with the custom countdown tool.
Frequently asked questions
When is my half birthday if I was born on December 31?
June 30, since June has only 30 days. Some people celebrate on July 1 instead.
How old am I in days?
Multiply your age in years by 365, add the days since your last birthday, and add one for each February 29 you've lived through. Or use the age calculator.
When do you turn a billion seconds old?
About 31 years, 8 months and 8 days after you're born.