One can achieve this knowledge in person, or by relying on another dependable, knowledgeable person, such as hearing the call to prayer (athan) from a knowledgeable muezzin (one who gives the call to prayer) in clear weather (One is not allowed to use one's personal reasoning in this case.

This is true if the caller to prayer is really educated and knowledgeable of the times of prayer.

As for modem callers, they rely on clocks, therefore one should not pray as soon as one hears the call to prayer, rather one should wait until he is certain that prayer's time has started.

Furthermore, when seeking prayer time, one should refer to more than one clock and one caller).

Furthermore, one may know the beginning of time by using one's personal reasoning (ijtihad).

If one is not capable of reasoning, he may follow a mujtahid (Generally speaking, if one is not capable of forming opinion (ijtihad) on matters of Islamic Law, he may follow one who is qualified to issue expert legal opinion (a mujtahid).

This is true if one is sighted.

As for one who is blind he must follow a mujtahid even if he is capable of reasoning.

If one prays without being certain, and without following a mujtahid who is certain, that the prayer's time has begun, then his prayer is not valid because the condition of knowing the beginning of the prayer's time has not been satisfied.

If one prays because he thinks, depending on his ijtihad, that the prayer's time has begun and then realizes that he has prayed ahead of time, then this prayer counts as a make up (qada') for a missed prayer of the same kind, or as a non-obligatory prayer.

Since Allah (G. B. H) says: {For such prayers are enjoined on believers at stated times}