On the Islamic calendar it is stated that the Asr salah/prayer begins at 16:49 but at the mosque it is scheduled at 16:15.
So is it permissible to pray before the actual beginning time?
There is a known difference of opinion among the schools as to when the 'Asr salat is supposed to start:
Depending on season and location, there can easily be an hour's difference between these two times. Either (or both) are valid, depending on which school you choose to follow.
According to Jafari Fiqh when an object’s shadow equals twice the height of the object it is the best time for the Asr prayer.
The Imamis consider the time when an object’s shadow equals its height as the end of the time of fadilah (honor) for the zuhr, and when it equals twice the height of the object as the time of fadilah for the ‘asr prayer.
References:
Prayer (Salat) According to Five Islamic Schools of Law Part 1: The Time Of Zuhr And ‘Asr Prayers