Your half-birthday is the day exactly halfway to your next birthday: six calendar months after the day you were born. To find it, add six months to your birthday. A birthday on 20 May gives a half-birthday of 20 November, and a birthday on 3 January gives 3 July. The half-birthday calculator does this for any date and also tells you how many days are left until the next one.
Why six months, not 182 days
There are two ways to define a half-birthday, and they can disagree by a day. Adding half a year as a fixed number of days lands on 182 days in a common year and 183 in a leap year, so the date shifts around. Adding six calendar months instead keeps the day of the month the same, which is easier to remember and stays put from year to year. This calculator uses the six-calendar-months method, so 20 May is always 20 November.
The leap-day case
Someone born on 29 February only has a real birthday every four years, which makes the half-birthday extra handy. Six months from 29 February is 29 August, and because August always has a 29th, the half-birthday works every single year with no special handling. It gives a leap-day baby a fixed date to celebrate in the three years out of four when 29 February does not exist.
The end-of-month case
The one case to watch is a birthday late in a long month. Six months from 31 August is February, which has no 31st, so the half-birthday clamps to the last day of the month: 28 February, or 29 February in a leap year. The same happens for 31 October, which maps to 30 April. The calculator applies this clamp automatically and shows the exact date for the year you are asking about.
Working it out by hand
- Take the month of the birthday and add six. If the total goes past 12, subtract 12 and it falls in the next calendar year.
- Keep the same day of the month.
- If that day does not exist in the new month, use the last day of that month instead.
For example, a birthday on 15 September: 9 plus 6 is 15, minus 12 is 3, so March, on the 15th. The half-birthday is 15 March. To skip the arithmetic and get the next date plus a countdown, open the half-birthday calculator. If you would rather count down to the real thing, the birthday countdown and the guide on how many days until your birthday cover that.
