Niddah 64 - Beginning of a Fixed Period
If a woman's period is accompanied by physical conditions as described earlier , the blood may still come at the beginning or at the end of this condition. If it usually comes at the beginning, and she discovered blood at a later time, it is presumed that it really came at the onset of the symptoms, and therefore all ritually pure foods that she handled since then are declared impure retroactively. However, if the blood usually appears at the end of the symptoms, all the foods that she handled prior are still ritually pure.
Rabbi Yose says that even hours count, and if, for example, she is used for her period to come at six hours on a certain day, it can be assumed every next time the blood also comes at the sixth hour. This has relevance to ritually pure food, but also to the couple's marital relations. For example, if she is accustomed to experience a discharge at sunrise, she is permitted to cohabit until sunrise. However, Rabbi Yehudah disagrees and says that only complete days count, so in our example, she is permitted to cohabit the entire day prior to her fixed period, but is forbidden with the coming of the night.
Art: Martin Johnson Heade - Sunrise