Shalom, A situation.
The woman started nidah on the 1st in the morning. Let’s assume it’s regular and it’s 5 days. The question is can you do hefsek taharah without having completed the fifth day? Or does it have to be after 5 full days?
The end of the fifth day is sufficient, and there is no need to wait a full five days. The hefsek however has to be made before sundown on the fifth day.