Birthday: April 18, 1971.

Day of Week: Friday

Days Until Next Half Birthday: 38

Days Since Last Half Birthday: 328

Age You'll Turn: 53.5

(Based on +6 month calculation. Click Here for +182.5 days calculation)

There are two common ways to calculate a half birthday.

+6 Months
The easy method is to just add 6 months to your current birthday and keep the day of the month the same.
e.g. If you were born on (Month 5) May 10, then your half birthday would be on (Month 5 + 6 Months = Month 11) November 10.

If you were born in a month with more days in it then the days in 6 months ahead has, then you would carry those days forward to the next month.
e.g. If you were born on (Month 5) May 31, then your half birthday would be on December 1. (Month 5 + 6 Months) would give you November 31, but since there's no November 31, you would carry the extra day foward to the next actual date, which is December 1.

+182.5 Days
The more exact method is to add 182.5 days (half a year or 183 days in a leap year) to your birthday. Generally you'll round up to 183 days (method used on BirthdayCalculator.com) to get to your half birthday.

The +182.5 Days method has more variability in it; if you subtract 183 days before your birthday, you'll end up with a different date than if you were just to add 183 days. You could also technically use either 182 days or 183 days in non-leap years.