Prayer: Time for Zuhr and Asr Prayers
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 717:
The time for zuhr and asr prayers is from zawal [i.e. the time after midday when the sun begins to decline] (known as the ‘legal midday’ (al‑ẓuhr al‑sharʿī))[1] until sunset (ghurūb). However, in the event that one intentionally (ʿamdan) performs the ʿaṣr prayer before the ẓuhr prayer, his prayer is invalid (bāṭil), except if this happens at the end of the prescribed time and there is scope for performing only one prayer, in which case if someone has not performed the ẓuhr prayer by then, his ẓuhr prayer is deemed to have become qaḍāʾ and he must perform the ʿaṣr prayer. If before this time someone mistakenly performs the whole of the ʿaṣr prayer before the ẓuhr prayer, his prayer is valid (ṣaḥīḥ), and he must then perform the ẓuhr prayer. And the recommended precaution (al‑iḥtiyāṭ al‑mustaḥabb) is that he should perform the second set of four rakʿahs with the intention (niyyah) to fulfil whatever his legal obligation happens to be (mā fī al‑dhimmah).
-Ayatullah Sistani, Practical Laws of Islam, Prayer (Salah)
The legal midday is defined as the passing of the midway point of the day. For example, if the day is twelve hours long, the legal midday is after the passing of six hours from the time of sunrise. If the day is thirteen hours long, the legal midday is after the passing of six and a half hours from the time of sunrise. [Author]