When is my half birthday?

When the month six months on has no 31st, the half birthday is its last day: March 31 becomes September 30, May 31 becomes November 30, October 31 becomes April 30 and December 31 becomes June 30. August 31 takes February 28, the last day of a common February, in leap years too. January 31 and July 31 keep the 31st.

February 28, every year. February has 28 days in a common year and the rule does not change in a leap year, so August 28, 29, 30 and 31 all share February 28 as their half birthday.

August 29: six months on, the same day of the month. Leap-day birthdays come round once in four years, but the half birthday is there every year.

Close, not exact. Six calendar months run from 181 to 184 days, depending on which months they cross and whether a February 29 falls in between; half a year is about 182.6 days.