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A half-birthday is a day approximately six months before or after the anniversary of a person's birth. It is sometimes marked by people whose birthday falls near major holidays, the celebration of which may overshadow celebration of the birthday.[1] It may also be marked by students whose birthday does not occur during the regular school year; a half-birthday allows a celebration with friends at school, with half a cake.[2]


There are two ways to calculate half-birthdays.

The easier but potentially less precise method is to take the number of the date of the birthday and advance the month by six: e.g. December 5 becomes June 5. Because not all months have the same number of days, this does not always work – for example, six months after an August 30 birthday would be February 30, which is nonexistent in the Gregorian calendar.

In the U.S., there is a 10% federal income tax penalty for making an early withdrawal from an IRA, 401K, annuity, or whole life insurance policy before age 59½, with some exceptions, and this is the method used. If it results in a nonexistent date, the last day of the month is used instead--even if it is a day or two short.[citation needed]

The more precise method is to add or subtract half the number of days in a year to the birth date. In the case of a common year, this would be 182.5 days. In leap years, the number of days would be 183. This method would lead to a March 1 or February 29 half-birthday for an August 30 birthday, depending on whether it's a leap year.[3]

Popular culture[edit]

At least three children's books have been written about half-birthdays:

  • Pomerantz, Charlotte (1984). The Half-Birthday Party. New York: Clarion Books. ISBN 0-89919-273-4.
  • Martin, Ann (1996). Karen's Half Birthday. New York: Scholastic. ISBN 0-590-69186-4.
  • Graham, Bob (2005). Oscar's Half Birthday. Cambridge: Candlewick Press. ISBN 0-7636-2699-6.

See also[edit]