2,099. Checking Days Late
Hilchos Issurei Biah 8:13
Let’s say that a woman didn’t check herself at her fixed time but she did check herself a few days later and found that she was unclean. In such a case, she is considered to be unclean retroactive to her fixed time when it comes to matters of ritual purity. She does not, however, render a man who was intimate with her retroactively unclean. Furthermore, she may only count from the time she saw blood. If when she checked she found herself clean, we assume that she is in fact clean.
Hilchos Issurei Biah 8:14
If a woman sees blood from a wound in her uterus, she is clean even if she sees the blood at the fixed time for her period to begin; the blood itself is also pure. This is because the practice of observing fixed days is of rabbinic origin, as will be discussed IY”H in Hilchos M’tamei Mishkav u’Moshav (in Sefer Taharah).