When did bleeding start?

In terms of calculating onot, sometimes it's unclear to me whether to consider my period as having started during the day or during the night. If a woman notices blood on underwear upon awakening in the morning, and has ongoing flow that day, should she count the period as having started during the daytime, or count it as having started during the night before (because of the blood seen upon waking up)? What about if a period starts with blood seen on a sanitary pad after shekiah (sunset), but which could have started earlier in the day and was not noticed--should that count as daytime or nighttime?
Thank you for your wonderful website!

Dear questioner,

Thank you for your question.

Thank you for the compliment. Where there is uncertainty about when the bleeding began - as is generally the case when one wakes up with a stain - the calculation of the onah is based on the time the bleeding is discovered. Thus, a woman who awoke after sunrise and found blood on her underwear would consider her period to have started during the day. A woman who noticed blood on a sanitary napkin after sunset, and did not feel the wetness during the day, would consider her period to have started at night.

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