counting the days to your period to determine whether you will have it for some important event (vacation, date, etc).
girl 1: Ready for our all-girl cruise next month?
girl 2: Yes! I just did some menstrual math, and I won't have my period until we get back!"
girl 2: Yes! I just did some menstrual math, and I won't have my period until we get back!"
by BillieGoat June 12, 2013