What is the ruling on prayer if one is not sure whether its time has begun?

It is not permissible to intentionally begin the prayer before verifying that its time has entered. Whoever does so has sinned, and their prayer is invalid because the entry of the time is a condition for its validity. If someone does so unintentionally, their prayer is a supererogatory (nafl) one, and they must repeat it.

If the intention is not being sure about the ihram (opening takbir) in it, then it is also invalid, because ihram in prayer is the act of entering it. If the worshipper is not certain that they pronounced the takbirat al-ihram, they have not entered the prayer at all, based on the Prophet's (peace be upon him) saying: "The key to prayer is purification, its consecration is the takbir, and its release is the taslim." That prayer is not considered valid unless the worshipper is someone who experiences frequent doubts.