Time for Zuhr and Asr prayers

737. If a stick, a pole, or anything similar to it, which acts as an indicator (shakhis) is made to stand on a level ground, its shadow will fall westwards when the sun rises in the morning, and as the sun continues to rise the shadow cast by the indicator will reduce in size. And in our cities it becomes smallest at the time of the commencement of Zuhr.
And as Zuhr passes the shadow cast by the indicator turns eastwards, and as the sun moves towards west the shadow gets longer. Based on this, when the shadow is the shortest, and it begins getting longer again, it is known that Zuhr has taken place. However, in other cities like in Mecca, the shadow disappears totally, so, when it reappears it indicates Zuhr.

738. The time for Zuhr and Asr prayers is from when the sun starts declining at midday till sunset. But, if a person intentionally offers Asr prayers earlier than Zuhr prayers, his prayer is void. However, if a person had not prayed Zuhr till the end of time, and the time left before Qadha allows only one Namaz to be prayed, he will first offer Asr prayers in time and then his Zuhr will be Qadha.
And if before that time a person offers complete Asr prayers before Zuhr prayers by mistake, his prayer is valid. But as a recommended precaution, he should treat that Namaz as Zuhr and should offer 4 more Rak'ats of prayers with the intention of relieving oneself of responsibility, if any (Ala mafi zzimmah).

739. If a person begins offering Asr prayers forgetfully before Zuhr prayers and during the prayers he realises that he has committed a mistake, he should revert his Niyyat to Zuhr prayers i.e. he should intend that from now onwards till the end of the prayers, it would be Zuhr prayers. After completing the prayers, he will offer Asr prayers.