Period Date Shift Planner
See how travel, exams or events line up against predicted period dates and how many days a clash would need to move.
About this tool
Period Date Shift Planner
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.
How to use Period Date Shift Planner
- 1
Open Period Date Shift Planner on AltFTool — it loads instantly in your browser.
- 2
Enter 'First day of your last period', 'Average cycle length (days)', 'Days of bleeding' and 'Cycles to project' (up to 12).
- 3
Set 'Event starts' and 'Event ends' for the trip, exam block or event you are checking.
- 4
The headline counts the overlap days, rows show how much earlier or later the period would need to start, and the 'Projected periods' table marks each cycle clash or clear — press 'Copy result'.
Why use Period Date Shift Planner
Overlap in whole days
Not just a clash flag — you see exactly which days of the event fall inside the predicted period.
Shift both directions
Shows how many days earlier or later the period would need to start to miss the event entirely.
Several cycles ahead
Projects up to twelve cycles so a booking months away can still be checked.
Common use cases
- Check whether a two-week holiday in September lands on a predicted period before you book.
- See if your exam week overlaps day 1 to day 3, when symptoms are usually heaviest.
- Plan a race, wedding or long flight around the projected bleeding window.
- Work out how many days a period would have to move to fully clear a fixed-date event before speaking to a clinician.
Frequently asked questions
How do I know when my next period will start?
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.
How accurate is period prediction?
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.
Can I delay my period for a holiday or exam?
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.
What if my cycles are irregular?
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.