Half birthdays are popular for babies (the six-month milestone) and for children – and adults – whose birthday falls during holidays. Birthdays on 29–31 August map to the end of February.
How to use the Half Birthday Calculator
- Date of birth
- Count from
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter your date of birth and the date to count from.
Tips
- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so “one month” is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.
Common mistakes to avoid
- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).
Inputs explained
| Field | What to enter |
|---|---|
| Date of birth | A date |
| Count from | A date |
What each result means
- Next half birthday = addmonths([Date of birth], 12×[Half birthday index] + 6)
- Falls on a = weekday([Next half birthday])
- Days until = [Next half birthday] – [Count from]
- Age you will be = [Half birthday index] + 0.5
Formula
Half birthday = birthday + 6 monthsNext half birthday = date of birth + (12n + 6) months, the first one after todayWorked example
Inputs — Date of birth: 2018-12-25; Count from: 2026-10-25.
Results — Next half birthday = Fri, June 25, 2027; Falls on a = Friday; Days until = 243; Age you will be = 8.5 years.
Frequently asked questions
When is my half birthday?
Six months after your birthday – 25 December becomes 25 June.
What if my birthday is 31 August?
The half birthday is the last day of February.
Why celebrate a half birthday?
Often because the real birthday falls during holidays or school breaks.
What formula does the Half Birthday Calculator use?
Half birthday = birthday + 6 months.