The Correct Time for Fajr and ‘Isha Prayers and Relying on Calendars
9-6-2021 | IslamWeb
Question:
السلام عليكم ورحمة الله وبركاتهWhat is the correct Fajr (and Isha) setting on our smart devices? Most mosques use 18 degrees but I've read that the twilight at that stage cannot be seen with the naked eye (astronimical twilight).
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
We have already explained in several Fataawa that the basic principle in determining the time of Prayers is the signs that the lawmaker [i.e. Allah] set up for that and not calendars, and that there is nothing wrong with relying on those calendars that are issued by trusted bodies.
With regard to the red twilight and its disappearance, it is possible to see it with the naked eye in principle, but this may not be possible in towns due to the abundance of lights. So, if you can know the time of the beginning of dawn and ‘Isha Prayers by seeing the true dawn at Fajr and the disappearance of the red twilight at ‘Isha, then that is good. If you cannot do that – and this is mostly the case of those who live in cities – and it is necessary to rely on one of those calendars, then act according to the calendar whose beginning time [for the Prayer] is late, because the Prayer is not valid until after being certain that its time has begun or predominantly thinking that it is the case.
Ibn Qudamah 
said in Al-Mughni:
“If one doubts about the beginning of the time of the Prayer, then he does not pray until he is certain of its beginning, or that he predominantly thinks that its time has begun.” [End of quote]
Based on this, if the beginning of the time of Fajr in one of these calendars is at 5:15, for example – and on another calendar its beginning time is 5:30 – then relay on this second calendar and not the first one. The same thing applies to the ‘Isha Prayer; if its time starts in one of the calendars at 6:15, for example, and on another calendar its time starts at 6:30, then rely on this second calendar and do not rely on the first, so that you perform the Prayer after you predominantly think that its time had begun.
For more benefit on the timings of Fajr and ‘Isha Prayers and relying on calendars to know the Prayer times, please refer to Fataawa 380553, 13950, 402688, 92311, 88333, 387526, 343828 and 357307.
Allah knows best.