Is it permissible for a woman to calculate the number of days of her menstrual period, and how is that done, especially if, at the beginning of her puberty, she did not consider yellowish or brownish discharges as menstruation due to ignorance?

Yellowish and brownish discharges are considered part of menstruation if they occur during the woman's customary days of bleeding. However, after purification (taharah), they are not considered anything, based on the statement of Umm Atiyyah: "We used not to consider the yellowish and brownish discharges after purification as anything." And for whoever thought them to be a state of purity, or was ignorant of this ruling, there is no blame upon her, for Allah Almighty says: "Allah does not burden a soul beyond its capacity."