But it’s a different length every time. Like one cycle might be 26 days, the next one 24 and so on. Unless you use a hormonal contraceptive I suppose, but who would want to risk the long term damages that causes.
The app I currently use can calculate it using the body temperature I enter and adjust the planned dates based on that. I’m pretty lucky to live in a first world country I guess because having to calculate this every day on my own sounds like a pain.