How to determine Asr time?
Refering to hadith xxx, Asr time begins either when the shadow is the same as the length of the object (denotes as one time shadow) or twice the length of the object (denotes two times shadow). However, note that for most of the times there always be shadow at noon. Therefore, the one time shadow and two times shadow should be calculated in addition to the length of the shadow at noon.
The formula is given in xxx. More elaboration on this topic is given in xxx and xxx.