Period Calculator

This is calendar arithmetic on a menstrual cycle. Give it the first day of your last period and how long your cycle usually runs, and it projects up to six future period dates, marks the estimated ovulation day fourteen days before the next period is due, and brackets the fertile window around it. A second mode answers the narrower question of whether a period is late, by counting the days between the date it was due and the date you ask about. Cycles drift by several days month to month, so these are estimates, not a contraceptive method and not a diagnosis.