Half Birthday: How to Find Yours (Even on the 31st)

Your half birthday is the date exactly six months after your birthday. If you were born on 15 June, your half birthday is 15 December. If you were born on 31 August, it falls on 28 February in most years and on 29 February in a leap year, because February has no 31st.

People celebrate half birthdays for many reasons: kids with summer birthdays who never get a party at school, couples marking six months, and babies reaching their six-month milestone. This guide shows how to work out a half birthday for any date, how to handle the tricky end-of-month and leap-year cases, and how a half birthday is different from simply counting 182 days.

How to calculate your half birthday

The standard method is to add six calendar months to your birth date:

1. Take your birth month and add six. If the result is more than 12, subtract 12.
2. Keep the same day of the month.
3. If that day does not exist in the new month, use the last day of that month.

So a 10 April birthday becomes 10 October, and a 20 November birthday becomes 20 May. Our age calculator uses calendar months in the same way when it shows your age in years, months and days.

Half birthday chart by month

Birth 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

The day stays the same unless it does not exist in the half-birthday month. That only happens for birthdays on the 29th, 30th or 31st, which are covered next.

End-of-month birthdays

Months have 28, 29, 30 or 31 days, so some birth dates have no exact match six months later. The usual rule, used by most date calculators, is to move to the last day of the shorter month:

Birthday	Half birthday
31 March	30 September
31 May	30 November
31 August	28 February (29 February in a leap year)
30 August	28 February (29 February in a leap year)
29 August	28 February (29 February in a leap year)
31 October	30 April
31 December	30 June

Only five 31st birthdays have to move: 31 March, 31 May, 31 August, 31 October and 31 December. A 31 January birthday keeps its day (31 July), and so does 31 July (31 January), because both of those months have 31 days. Birthdays on 29 and 30 August also move, to the end of February. Some families prefer to celebrate the next day instead, for example 1 March for a 31 August birthday. Either choice is fine for a party; just be consistent.

Leap-year birthdays

If you were born on 29 February, your half birthday is 29 August every year, because August always has 29 days. That makes it one of the few dates a leapling can celebrate on the exact day every year. Our guide to leap year age calculation covers how leap-day birthdays are counted for legal age and which day people usually celebrate in common years.

Half birthday vs 182 days

Six calendar months is not always the same as half a year counted in days. A common year has 365 days, so half a year is 182.5 days, and a leap year has 366 days. The number of days between your birthday and your half birthday depends on which months you pass through:

• From 31 August 2026 to 28 February 2027 is 181 days.
• From 16 October 2026 to 15 April 2027 is also 181 days, while 16 October to 16 April is 182 days.
• Spans that include July and August, two 31-day months in a row, tend to be longer.

For a half birthday, the calendar-month method is the one people expect. If you need an exact day count for a contract, a medical schedule or a project, use the age in days calculator or a date tool such as timeanddate’s add to or subtract from a date calculator, and say which method you used.

Worked half birthday examples

Here are five birthdays worked through step by step, using the three-step method above:

1. Born 15 June 2012. June is month 6; 6 + 6 = 12, December. The 15th exists in December, so the half birthday is 15 December.
2. Born 3 November 1990. November is month 11; 11 + 6 = 17, minus 12 = 5, May. The half birthday is 3 May.
3. Born 31 May 2005. May + 6 months = November, which has 30 days. The half birthday is 30 November.
4. Born 30 August 2019. August + 6 months = February. In 2027, a common year, the half birthday is 28 February; in 2028, a leap year, it is 29 February.
5. Born 29 February 2004. February + 6 months = August, which has a 29th. The half birthday is 29 August every year.

How to find a half birthday in Excel or Google Sheets

If you are working out half birthdays for a whole class, team or family list, a spreadsheet saves time. Both Excel and Google Sheets have an EDATE function that adds whole months to a date:

• Put the birth date in cell A2.
• In B2, enter =EDATE(A2,6) and format the cell as a date.

EDATE keeps the same day of the month and, when that day does not exist, returns the last day of the target month, which matches the end-of-month rule above. So a 31 August birth date returns 28 or 29 February, depending on the year in the cell. To get this year’s half birthday rather than the one six months after birth, build the date with this year first, for example =EDATE(DATE(2026,MONTH(A2),DAY(A2)),6), and check leap-day birthdays by hand. For other age formulas, see our chronological age in Excel guide.

Common half birthday mistakes

• Adding 182 days instead of six months. That can land a day or two away from the calendar date people expect.
• Forgetting the month-end rule. There is no 31 September or 30 February, so those birthdays need the last day of the month.
• Using the wrong year for 29 February. A 31 August birthday only reaches 29 February in a leap year, such as 2028.
• Counting from the wrong birthday. A half birthday follows your most recent birthday, so in October 2026 a 15 June birthday points to 15 December 2026.

Half birthdays for babies: the six-month milestone

Parents often mark a baby’s half birthday because six months is a real milestone. Pediatric check-ups and developmental screening are often scheduled by age in months, so knowing the exact six-month date helps. For babies born early, health professionals often use corrected age, which subtracts the weeks of prematurity, when tracking development. Our adjusted (corrected) age calculator and guide to chronological vs adjusted age explain the difference.

How old are you on your half birthday?

On your half birthday you are exactly your age plus six months, for example 9 years and 6 months, often written as 9½ or 9;6 in assessment records. That year;month notation is the same one used on our months to years guide, so 9;6 means 114 months. If you need your age down to the day, the age calculator shows years, months and days together.

Ideas for celebrating a half birthday

Half birthdays are low-key by tradition, which is part of the fun. Popular ideas include:

• Half a cake. Bake a round cake, cut it in half, and stick the halves together for a half-moon shape.
• A school celebration for summer-birthday children whose real birthday falls during the holidays.
• A six-month check-in for couples, roommates or new jobs.
• A “halfway there” savings goal for kids saving toward something by their next birthday.
• A letter to your future self, opened on your next birthday.

Half birthdays in a classroom

Teachers with summer-born students often schedule half-birthday celebrations so every child gets a turn during the school year. A simple way to plan:

1. List every student’s birthday.
2. Mark any that fall in June, July or August, or whenever school is out.
3. Add six months to get each half birthday, using the end-of-month rule above.
4. Put the dates on the class calendar.

If you are working out ages for a whole class or group, our guide to cumulative age shows how to add ages together, and precise age in months, days and weeks explains why the counting method matters.

Related age milestones

Half birthdays sit alongside other date milestones people like to track: 1,000 days old, 10,000 days old, or the month you turn 65 for Medicare, covered in our recent guide to when you turn 65. And if you want to know which generation a birth year belongs to, see our new guide, what generation am I.

Checklist: finding a half birthday

• Add six to the birth month, wrapping past December.
• Keep the same day of the month.
• If that day does not exist, use the last day of the month.
• Leap-day birthdays: the half birthday is always 29 August.
• Need an exact day count instead? Use a days calculator and say so.
• Put the date in your calendar so you do not miss it.

The same counting method works for a holiday countdown.

The same counting works for a countdown to Christmas.

Frequently Asked Questions

Next birthday puzzle: what day of the week was I born?