It is permissible to use timetables that show the times of prayer so long as it has not been proven that they are wrong. It has been proven that many of them are wrong – if not all of them – with regard to Fajr prayer, and some of them are wrong with regard to ‘Isha’. With regard to Maghrib the mistakes are slight, because it is easy for most people to know what the right time is, and to figure out the mistakes in them, by watching the sun themselves.
Whatever the case, the sign of sunset which is when the fasting person breaks his fast and the time for Maghrib prayer begins is when the disk of the sun actually disappears behind the horizon, not behind mountains or buildings.
The Sahaabah (may Allaah be pleased with them) stated the time of Maghrib as being when the sun disappears in the veil (of night), and they used different phrases to describe that. Some of them said “the sun set” and others said “(the sun) had hidden in the veil (of night)” (cf. Saad 328:32) or “the sun disappeared”. These phrases all convey the same meaning, which is the total disappearance of the sun’s disk.
You do not have to climb up the mountain or to any high place, rather what is meant by sunset for the place where you live is when it disappears below the horizon; when it just disappears behind the mountains, this is not regarded as sunset.
As you cannot see the sun at the time of sunset because of the mountains, you can know the time of its setting from another sign which was mentioned by the Messenger (peace and blessings of Allaah be upon him), which is when darkness comes from the east.
Al-Bukhaari (1954) and Muslim (1100) narrated that ‘Umar ibn al-Khattaab (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “When the night comes from here (the east) and the day departs from here (the west), and the sun sets, then it is time for the fasting person to break his fast.”
With regard to the words of the Prophet (peace and blessings of Allaah be upon him): “The night comes and the day departs and the sun sets”, the scholars said: Each of these three implies the other two, and he mentioned all of them together, because a person may be in a valley and the like, where he cannot see the sunset, so he has to rely on the coming of darkness and the disappearance of daylight. And Allaah knows best. End quote.
If this is not possible either then there is nothing wrong with relying on timetables, because at least they state the likely time of the prayer, so long as it has not been proven that they are wrong.
And Allaah knows best.