Clearblue Fertility Monitor

Has anyone used this before? I was reading that you have to enter “m” on the first day of your period but I’m almost done with mine and I already purchased this. Can I still use it or do I have to wait next cycle if I happen to not conceive this cycle?! It doesn’t say anything about being able to enter a past period...(All prayers towards are march conception, though!!! 🙏🏽🥰🤞🏽)