I say, and with God's success: It becomes invalid if performed before sunrise when the sun rises; because the part that coincides with the performance is a reason for the obligation of prayer, and the last time for the afternoon prayer is a deficient time, as it is the time of worship of the sun, thus it must be deficient. If it is performed, it is performed as it is required. If corruption occurs at sunset, it does not become invalid. In the case of dawn, all of its time is a complete time; because the sun is not worshipped before its rising, thus it must be complete. If corruption occurs at the rising, it becomes invalid; because it was not performed as it should have been. And because the afternoon prayer extends to what is the time of prayer in general, unlike dawn; because at the rising there is an entry into dislike, and at sunset, there is an exit from it. See: Tabyin al-Haqaiq 1: 85-86, and al-Tawdiḥ 1: 206, and Takhriq al-Tanqiḥ by Ibn Kamal Pasha 1: 128, and Mir'at al-Usool 1: 134-135, and Sharh al-Manar by Ibn Malik p59-60, and Sharh al-Manar by Ibn al-Ayni p60, and al-Talwiḥ 1: 207, and God knows best.