What is the advice for the mu'adhhdhins regarding determining the time for the adhan?

The questioner says in his following question: The mu'adhhdhins do not adhere to a single time for the five daily prayers, especially the Fajr prayer, despite there being a standard time schedule for each city. What do you advise them to do? May Allah guide you.
The answer: We advise the mu'adhhdhins to always adhere to the time for the Zuhr, 'Asr, Maghrib, 'Isha', and Fajr prayers. This is obligatory upon them. It is obligatory for them to adhere to the time and to ascertain the entry of the time. They should not follow the calendars blindly, because they contain errors and mistakes. They should take care and be diligent in regulating the time. The zenith is known by the sun, the sunset by the setting of the sun for Maghrib, 'Isha' by the disappearance of the twilight, and Fajr by the appearance of the dawn. They should ascertain the time if they are able to do so. If they are not able to do so, they should verify the calendar and ascertain its accuracy. If the calendar is early, they should delay it by an additional five or ten minutes, according to what becomes clear to them through experience, and so on until they regulate the matter with something clear. If the calendar is late, they should measure the delay and advance before it. If the calendar is early before the time, they should measure the deficit in the calendar, add it to their calculated time, and then call the adhan with certainty. The result is that it is obligatory for them to ascertain the time with all care, because the people are under their responsibility, and they are trustees regarding the prayer and breaking the fast in Ramadan. It is obligatory for them to ascertain the time and take care of it. It is clear, with the praise of Allah, if they take care of it. The time for Zuhr enters when the sun passes the zenith, after the shadow of the zenith. The time for 'Asr enters when the shadow of every object becomes equal to its length, after the shadow of the zenith. The time for Maghrib is when the sun sets. The time for 'Isha' is when the red twilight in the direction of the west disappears. The time for Fajr is when the true dawn appears, which is the horizontal dawn that spreads across the horizon. This is the true dawn. It is obligatory for them to ascertain these things by sight or by a verified calendar that has been studied, taken care of, and known to correspond to reality. Yes.