Regarding your current question, the personality of Prophet Muhammad (sws) is the only source of religion now till the end of this world. It is from Him (sws) we received the whole of religion in two forms namely: Quran & Sunnah. Both of them are transmitted to Muslim Ummah by a consensus (ijma) of Sahaba (Companions) of Prophet from generation-to-generation.

All the ibadah including, Hajj, Salah, Zakah, Saum (fasting), Animal Sacrifice on the day of Eid etc. were initiated by the Prophet (sws) himself and have been in practice even today without any discontinuation on the part of Muslim Ummah. It is through this Sunnah from where we got the law of fasting including its law, its time and other application matters. Quran sometimes mentions particular aspects of these Ibadah, sometimes to emphasize on them, sometimes to give any relaxation and sometimes to clear any big misunderstanding; but all of this is done under the pretext that all these Ibadah were already practiced by Muslims. Quran is not the initiator of those Ibadah.

Like the prayer, the fast too was well-known to the Arabs. They were fully aware of its religious status and its details viz-a-viz its bounds and limits. Consequently, when the Quran directed them to fast, these bounds and limits were not unknown to them: in fact, the words in which this directive was given, shows that they should observe it as an obligatory ritual which they knew as an age old ritual and an age old Sunnah of the prophets. The Prophet (sws) and his Companions (rta) fasted on this very basis and Muslims after them are following this practice generation after generation. Viewed thus, the source of the fast is also the consensus and tawatur (practical perpetuation) of the Muslims. The only thing that the Quran did was to make the fast an obligatory ritual, stipulating certain principles of lenience for the sick and for the travellers and to answer certain questions which were raised by the Muslims regarding the fast.

Therefore Quran while clarifying certain questions and misconceptions about fasting, mentioned:

أُحِلَّ لَكُمْ لَيْلَةَ الصِّيَامِ الرَّفَثُ إِلَى نِسَآئِكُمْ هُنَّ لِبَاسٌ لَّكُمْ وَأَنتُمْ لِبَاسٌ لَّهُنَّ عَلِمَ اللّهُ أَنَّكُمْ كُنتُمْ تَخْتانُونَ أَنفُسَكُمْ فَتَابَ عَلَيْكُمْ وَعَفَا عَنكُمْ فَالآنَ بَاشِرُوهُنَّ وَابْتَغُواْ مَا كَتَبَ اللّهُ لَكُمْ وَكُلُواْ وَاشْرَبُواْ حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ مِنَ الْفَجْرِ ثُمَّ أَتِمُّواْ الصِّيَامَ إِلَى الَّليْلِ وَلاَ تُبَاشِرُوهُنَّ وَأَنتُمْ عَاكِفُونَ فِي الْمَسَاجِدِ تِلْكَ حُدُودُ اللّهِ فَلاَ تَقْرَبُوهَا كَذَلِكَ يُبَيِّنُ اللّهُ آيَاتِهِ لِلنَّاسِ لَعَلَّهُمْ يَتَّقُونَ

“It is lawful for you to lie with your wives on the night of the fast; they are apparel to you as you are to them. God knew that you were being dishonest with yourselves. He has relented towards you and pardoned you. Therefore, [without any hesitation] you may now lie with them and [without any hesitation] seek what God has ordained for you. And eat and drink until the white thread of the dawn is totally evident to you from the black thread of night. Then complete the fast till nightfall and do not lie with them [even at night] when you are stationed for itikaf in the mosques. These are the bounds set by God: do not approach them. Thus He makes known His revelations to mankind that they may attain righteousness“. (Quran 2:187)

And in matters of Ibadah (‘Tabbudi amoor’), we cannot use ijtihad or reasoning to change a directive based on presence/absence of certain conditions, unless they are explicitly mentioned in the directive. In the absence of any such explicit condition, all matters of ibadah should be adhered to in form as given by the Prophet (sws) Himself (sws), either through Quran or Sunnah.

Please refer to the followng responses of Ghamidi Sahab as well to understand the principle of Ibadah (‘Tabbudi amoor’)

Discussion 34437 • Reply 44377

Discussion 34437 • Reply 44909

Regarding Sehri end time, please see:

Discussion 68439

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Regarding Aftar time, please watch:

https://youtu.be/LNfbz5Xih7o