Prayer: Impediments to Ascertaining Prayer Time
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 730
If due to a personal impediment, such as blindness or being imprisoned, a person cannot perform prayers at the start of their prescribed time on account of being unable to attain certainty in the time having set in, he must delay his prayers until he is certain or confident that the time has set in. And based on obligatory precaution, the same applies if the impediment to one attaining certainty in the time having set in is due to non-personal hindrances, such as clouds, dust, and similar things.