The Salat, other than the daily Salat do not have a specific time (to be read), rather they are related to a specific time and event which makes them become Wajib. For example, SalatulAyat is related to events like earthquakes, solar eclipses, lunar eclipses and other natural occurrences. On the other hand, SalatulMayyit, becomes Wajib when a Muslim passes away.
If the complete Salat is read before its time has set in, either intentionally or unintentionally, that Salat will be void. If the Salat is read in is specific time, then it is referred to as Salatul -Ada in Islamic terminology, and if it is read after the time has elapsed, then it is referred to as SalatulQadha.
A person must read his Salat in its specified time, and if one intentionally does not pray it in this time, then he has committed a sin.
It is Mustahab that a person read his Salat right when the time sets in, and the closer that it is read to when it becomes time, the better it is. However, if it is delayed for a good reason, such as delaying the Salat so it can be read in Jama’at, then this is better.
If the time for Salat is so short that if the person prays the Mustahabbat of the Salat, a part of the Salat will be performed after it’s time, then he must leave out the Mustahabbat. For example, if one wants to pray the Qunut, but the time will pass for the Salat, then one must not perform the Qunut.
A person must read Salatul‘Asr after SalatulZuhr and Salatul‘Isha after SalatulMaghrib, and if one intentionally prays Salatul‘Asr before SalatulZuhr or Salatul‘Isha before SalatulMaghrib, then the Salat will be void.