Half Birthday Calculator

DateMath adds 6 calendar months to the date of birth, then whole years, so the half birthday keeps the day number of the birthday: born November 17, the half birthday is May 17 every year. When the month 6 months on is shorter, it takes that month's last day, so a birthday on August 31 has its half birthday on February 28, or 29 in a leap year, and one on March 31 on September 30. Quarter and three-quarter birthdays are 3 and 9 months after the birthday, worked out the same way.

• The Gregorian calendar. DateMath treats every date as a Gregorian date, with its leap-year rule: a year divisible by 4 is a leap year, except century years not divisible by 400, so 2000 had a February 29 and 2100 will not. Source: Gregorian calendar.

DateMath's date math is tested against more than 100 cases worked out by hand. Every worked example and table on the site is computed with the same code the calculators run, so a page and its calculator can't disagree.

Your half birthday is the day you are exactly half a year older than on your birthday: 6 calendar months after it, on the same day number. Born on November 17, your half birthday is May 17 every year. The calculator works out what’s my half birthday from a date of birth, with the weekday it falls on and the days until it, and answers when is your half birthday for anyone else just as quickly.

The method, step by step

Born November 17, 1990, counting from January 1, 2027:

1. Add 6 months to the birth month: November + 6 months is May.
2. Keep the day number: the half birthday is May 17.
3. Take the first May 17 on or after the date you count from: Monday, May 17, 2027, when the age turns 36½.
4. Count the days: 136 days from January 1, 2027.

DateMath adds the months to the date of birth itself every year, so a month end never drifts: the half birthday of August 31 is February 29 in a leap year and February 28 in the others.

Find your birth month and read across: the second column gives the half birthday of the 15th, and every other day number works the same way. The third column gives the last day of the month, where the shorter month 6 months on moves the half birthday to its own last day.

Six calendar months, in DateMath’s count and in most people’s. A birthday year has 365 or 366 days, so its exact middle is 182 or 183 days after the birthday, while 6 calendar months is 181 to 184 days depending on the months in between. The two answers are often the same day and never more than a few days apart:

For a half birthday counted in days, add 182 days to the birthday in the date calculator.

The same rule gives the other fractions of a year: a quarter birthday calculator adds 3 months to the birthday and a three-quarter birthday 9 months. Born July 31, 2017, the next quarter birthday after January 1, 2027 is October 31, 2027 (Sunday), turning 10¼, in 303 days, and the next three-quarter birthday April 30, 2027 (Friday), turning 9¾, in 119 days.

That last one is the 9¾ birthday calculator answer for this child, the day a reader of Harry Potter might mark after Platform 9¾. For anyone else, enter the date of birth and pick a date after their 9th birthday, such as the 9th birthday itself, as the date to count from.

• Birthdays near the holidays. A birthday close to Christmas or New Year gets its own day in June or July.
• Summer birthdays. Children born in July or August often celebrate at school on the half birthday, when classmates are around.
• Babies. The 6-month mark is the first milestone parents photograph; the age calculator gives the age in months and weeks too.
• Couples. A half-year mark of a relationship is a monthly anniversary: the anniversary calculator gives it, with the 100th and 1,000th days.

For a countdown to the day, enter the half birthday in the day countdown and tick “Every year”. February 29 birthdays follow the rule of the age calculator, and the leap year calculator counts their real birthdays; for other special birthdays, see the golden birthday calculator.