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About this tool
See how travel, exams or events line up against predicted period dates and how many days a clash would need to move.
The Period Date Shift Planner projects your next period start dates from your last day 1 and your average cycle length, then measures exactly how many days they overlap a trip, exam block or event you have coming up. It uses plain calendar arithmetic — period n starts on your last period start plus n times your cycle length, and bleeds for the number of days you enter — and reports the overlap along with how many days the clash would need to move to clear the event either way.
Open Period Date Shift Planner on AltFTool — it loads instantly in your browser.
Add your input to the workspace.
Adjust the options until the result looks right.
Copy or download the output and put it to work.
Not just a clash flag — you see exactly which days of the event fall inside the predicted period.
Shows how many days earlier or later the period would need to start to miss the event entirely.
Projects up to twelve cycles so a booking months away can still be checked.
Add your average cycle length to the first day of your last period. If your last period started on 1 July and your cycle averages 28 days, the next starts around 29 July and the one after around 26 August. Calendar prediction assumes regularity, so treat the date as a window of a few days rather than a fixed appointment.
It is only as regular as your cycles are. A cycle described as regular still varies by several days, and the medically usual range is 21 to 35 days, so a prediction three or four cycles out can easily be a week off. Illness, travel, stress and big changes in training or weight all move dates.
It can be done, but only with prescribed medication — a progestogen such as norethisterone, or running combined pill packets together — and it needs a doctor or pharmacist to check it is suitable for you. This planner deliberately shows only the clash and the size of the shift; the decision and the prescription belong with a clinician.
Calendar projection is much less useful when cycle length varies by more than a few days, and it is not appropriate as contraception at all. Use the longest and shortest cycle you have recorded to see a best and worst case, and speak to a clinician if cycles are consistently shorter than 21 days or longer than 35.