Verbatim · Ayatollah A. Sistani
Three days is considered as 72 hours from the commencement of bleeding, and ten days as 240 hours. However, if for instance, bleeding commenced at 10:00 pm on Saturday (bleeding starts at night not at day), then one is not required to wait till 10:00 pm on Tuesday, rather on sunset of Tuesday will be the completion of three days, and also the sunset of the following Tuesday will be the completion of 10 days.
Ayatollah A. Sistani sistani.org Q&A #26307 other marjas may differ
This is the kind of detail the engine inside the Tuhr app handles for you. Once your start time is logged, the boundaries are computed automatically per your marja’s day-boundary convention.