Is the fast of one who felt wetness in the afternoon and then saw blood at 'Isha' valid?

If the blood exits before Maghrib, the fast is invalidated, and the day must be made up if the fast was obligatory. However, if you are unsure about the time of its exit and it is possible that it exited after Maghrib, then the fast is judged to be valid, because "the default rule for every incident is to estimate it by the nearest time."