Timing of Hefsek Taharah
However, the seven blood-free days may not begin before the sixth day of her niddah status (see Five-Day Minimum). Therefore, a woman ordinarily should not perform the hefsek taharah before the fifth day - even if she is convinced that her bleeding has ceased. If she would find it difficult to do a hefsek taharah on the fifth day, she may do it on an earlier day, leaving a gap between the hefsek taharah and the beginning of the shivah neki'im.
The hefsek taharah must be performed during the daylight hours, defined in halacha as between sunrise and sunset. The proper time is in the late afternoon shortly before sunset, but when necessary, the examination can even be done in the morning. It is invalid when performed in the morning only if she experienced a flow for only part of one day and wants to do a hefsek later that day (e.g. she had a flow during the shivah neki'im). In that case, it must be done in the late afternoon before sunset.
Busy women who have a tendency to forget, especially on short winter afternoons, and who don't find these exams painful or troublesome should do one in the morning and repeat it in the afternoon. In this way they have a "backup" in case of forgetting. On Erev Shabbat one should check before candlelighting even if the community starts Shabbat early; however as long as one checks before sunset, the exam is valid.
A woman who missed sunset by a few minutes should immediately perform the examination, note the exact time, and then consult a rabbi to determine whether it was valid.