في الإسلام، متى يبدأ الصوم؟

In Islam, the daily fast during the month of Ramadan extends من الفجر إلى المغرب (from dawn to sunset). This is established in the Quran in Surah al-Baqarah (2:187):

"وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الْأَبْيَضُ مِنَ الْخَيْطِ الْأَسْوَدِ مِنَ الْفَجْرِ ثُمَّ أَتِمُّوا الصِّيَامَ إِلَى اللَّيْلِ"

This verse instructs Muslims to eat and drink until the white thread of dawn becomes distinct from the black thread (i.e., until the Fajr prayer time begins), and then to complete the fast until nightfall (i.e., until the Maghrib prayer time).

The fast therefore begins at الفجر (dawn/Fajr) and ends at المغرب (sunset/Maghrib). It does not begin at sunset (that is when the fast ends and eating resumes), nor after Asr or after Isha - those times fall either within or outside the fasting period but are not its starting point.