Answered by Shaykh Amjad Rasheed
Is it permissible to practically rely on a prayer timetable or must one independently evaluate whether the time has entered?
Yes, it is permissible to rely upon a prayer timetable if it was produced by someone well-versed in astronomy. Based on this, if one has a reliable clock, it is permissible for one to look at the prayer’s start time according to the timetable and then check one’s clock. If the times agree, one can pray based on this and one is not obliged to independently evaluate [m: that the time has entered].  This is what has been codified in the books and is the position of the exacting Shafi’i scholars of Hadramawt. Furthermore, our shaykh, the erudite scholar and exacting jurist, Muhammad Ali Al-Khateeb (Allah perpetuate his benefit) has issued this fatwa to me.
 For a number of reasons, including year-to-year variations in prayer times, large metropolitan areas covered by a single calendar, and inaccurate clocks, it is best to wait several minutes after the time listed on a prayer timetable to ensure the time of the prayer has entered. For fasting, it is safest to stop eating several minutes before the time listed for fajr.