How this calculator works
Six calendar months are added to the birth date, keeping the same day number where that day exists. A birthday late in a month can therefore land on the last day of a shorter month, such as August 31 becoming February 28 or 29. The next occurrence is the half birthday in the reference year, or in the following year when it has already passed.
Formula
half birthday = birth date plus 6 months, with the day clamped to the length of the target month. next half birthday = the half birthday in the reference year if it is not before the reference date, otherwise the same in the following year. Days away = O(next) - O(reference), using day numbers.
Worked example
A birth date of 1990-05-20 gives a half birthday of 1990-11-20. On 2026-11-01 the next half birthday is 2026-11-20, 19 days away, while the next ordinary birthday is 2027-05-20, 200 days away.
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Frequently asked questions
Is a half birthday exactly 182 or 183 days after the birthday?
Not necessarily. Six calendar months can contain 181 to 184 days depending on which months they cross. The page adds months on the calendar rather than a fixed number of days, which is what most people mean by a half birthday.
What happens for a February 29 birthday?
The birthday itself is placed on February 28 in a common year, and the half birthday is six months later on August 28. In a leap year the half birthday is August 29.
Why celebrate a half birthday?
It gives a second small celebration halfway through the year, which is popular for children with summer birthdays or for people whose real birthday falls in a busy season. The date arithmetic is independent of the reason.
Does the time of day matter?
No. The page uses civil dates only, so the result is a date, not a moment. The half birthday does not inherit a birth time.