Half Birthday Calculator

1. Enter the date of birth.
2. Keep the default month-end rule or pick another. It only matters for the 29th to 31st, or if you want the exact 182-day version.
3. Read the next half birthday, the countdown and the age. Pick a "count from" date to plan ahead or check a past year.

A half birthday is the date halfway between two birthdays, usually taken as the same day number six months later. A child born on July 15 has a half birthday on January 15.

Families mark it for babies at six months, for children whose birthday falls in the holidays or in summer break, and as a mid-year check-in. It has no legal meaning, so no law or standard defines it. That is why this page states its convention and lets you switch.

A half birthday is one kind of date milestone. The guide to half birthdays and milestone dates covers others, such as 1,000 days old, and the anniversary calculator counts years, months and days since any event.

The calculator adds six calendar months to the birthday. When the target month is too short, it moves to the last day of that month.

1. Move the month forward by six: January to July, August to February, December to June.
2. Keep the same day number when that month has it.
3. When it does not, as with August 31 to February, use the last day: February 28, or February 29 in a leap year.
4. Repeat for each year to find the next half birthday on or after the count-from date.

Why the Last-Day Rule

The same question comes up in law for periods counted in months. EU Regulation No 1182/71, Article 3, as published on legislation.gov.uk, says that when the expiry day does not occur in the last month, the period ends on the last day of that month. The default follows that rule, so August 31 plus six months is February 28 or 29. Some people prefer March 1, and the next-day option gives that.

The Exact 182.62-Day Alternative

Under the Calendar (New Style) Act 1750, 97 of every 400 years are leap years. That makes 146,097 days in 400 years, an average year of 365.2425 days and a half year of 182.62 days. The exact option adds 182 whole days to the birthday, the day on which that midpoint ends for someone born at the start of the day. It can differ from the calendar rule by 1 or 2 days.

All examples count from September 30, 2026.

The August 31 case in detail: six months after August 31 is "February 31", which does not exist. The default gives February 28, 2027. The next-day rule and the exact 182-day option both give Monday, March 1, 2027. In 2028, a leap year, the default gives February 29. The old version of this tool let the date overflow into March 2 or 3, which matched none of these rules.

Only a few things change the answer, and the tool shows each one.

Birthdays on the 29th, 30th or 31st

Six months after the 29th to 31st of August lands in February, and six months after March 31, May 31, October 31 or December 31 lands in a 30-day month. These dates need a rule.

Leap Years

A February half birthday is the 28th in common years and the 29th in leap years. A February 29 birthday has its half birthday on August 29 every year, because August always has 29 days. See February 29 birthdays in common years for the full birthday question.

Which Convention You Pick

The calendar rules keep the same day number most years. The exact option keeps the same length, 182 days, but lands on a different day number.

Time of Birth

The tool counts whole days. Someone born late in the evening reaches the exact 182.62-day point on the following day.

The two methods agree for most birthdays. They differ when month lengths do not balance out.

Pick the calendar rule for parties and cards, because people expect the same day number. Pick the exact option for a day count, such as a baby's growth diary.

Holiday and Summer Birthdays

A child born in late December or during summer break can celebrate at school on the half birthday instead.

Baby Milestones

Find the six-month date for photos or a small party. The age line shows "6 months old" for the first one.

Planning Ahead

Set a count-from date to see the weekday of a future half birthday. To count forward from any date, use the date calculator.

1. Letting the Date Overflow

Some date code turns "February 31" into March 2 or 3. That matches no convention people use.

2. Adding 6 Months of 30 Days

180 days falls short of half a year. The real gap after the birthday is 181 to 184 days.

3. Forgetting Leap Years

A February half birthday moves between the 28th and 29th.

4. Counting the Birthday Itself

The countdown is a date difference. The day before a half birthday shows 1 day, not 2.

5. Mixing Time Zones

Dates here are handled as calendar dates in UTC, so the day never shifts for visitors in other time zones.

The date math is exact for any Gregorian date. The only judgment is the month-end convention, which you can choose.

Add six months to your birthday and keep the day number. Born on April 3, your half birthday is October 3.

It is February 28, or February 29 in a leap year, under the default rule. Some people use March 1 instead, and the tool shows both.

It is June 30 under the last-day rule, because June has 30 days. The next-day rule gives July 1.

It is August 29 every year. August always has 29 days, so no rule is needed.

Not quite. The average year is 365.2425 days, so half is 182.62 days. Calendar half birthdays fall 181 to 184 days after the birthday.

Enter your birth date and read the countdown. From September 30, 2026, a July 10 half birthday is 283 days away.

Yes. Set the "count from" date to any day, and the tool shows the next half birthday after it with its weekday.

Some date code lets February 31 roll over into March. That is a programming side effect, not a convention anyone uses.

It is your age plus one half. A child born January 10, 2014 is 12 and a half on July 10, 2026.

No. Dates are processed in your browser and are not sent to us. Save keeps a result only in this browser.

• Calendar (New Style) Act 1750, section 2 (legislation.gov.uk; century years are common years except every fourth hundredth year).
• Regulation (EEC, Euratom) No 1182/71, Article 3 (legislation.gov.uk; a period in months ends on the last day of the month when the matching day does not occur).
• ISO 8601 date and time format (International Organization for Standardization; the YYYY-MM-DD order).