Is the sighting of the crescent moon considered before or after sunset, and is the sighting valid if the crescent moon disappears before sunset, or is its persistence after sunset a condition, and is there a scholarly disagreement regarding the time of sighting?

If the crescent moon is sighted during the day, it belongs to the coming night. This applies whether it is on the 30th of Sha'ban, in which case there is no obligation to fast (Imsak), or on the 30th of Ramadan, in which case there is no obligation to break the fast (Iftar). This is the well-known position among the four madhhabs, regardless of whether the sighting occurs before or after Zawwal (midday). Some scholars, however, are of the view that if it is sighted before Zawwal, it belongs to the past night. If the crescent moon is sighted on the 30th day before sunset and then disappears before sunset, that night marks the beginning of the new month, and a sighting after sunset is not required. However, if it is sighted on the 29th day before sunset but not seen after it, the Malikis consider that sighting sufficient to establish that night as the beginning of the month, whereas the Shafi'is believe it is not sufficient unless it is also sighted after sunset.