Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states: Before breaking our fast, it is our duty to ascertain as best as we can, using the available means, that the sun has indeed set. In the modern world, especially in cities, we don’t have many excuses not to use the timetables that are based on precise scientific calculations where the margin of error is rather slim.

If, however, after having done one’s best, a person were to make a genuine mistake and break the fast before the sun set, then he must abstain from eating and drinking as soon as he has realized the mistake and thus complete the fast. If he realized the mistake only after the sun had already set, then he need not worry so long as it was not a deliberate act on his part. In both of the above cases, he need not redo the fast thus broken.
What we have stated above is the preferred view held by many scholars and jurists of Hadith and fiqh. It has been inferred from the sound precedents of the Prophet (peace and blessings be upon him) as well as his Companions. There is, however, a second view held by some jurists and scholars: According to them, one must redo the fast later; they consider this to be the safest route to follow in matters of worship.
The advocates of the first viewpoint to the authentic report that during the time of the Prophet (peace and blessings be upon him) some of his Companions broke their fast presuming that the sun had set, and later on the sun appeared from behind the clouds; there is no mention anywhere that the Prophet (peace and blessings be upon him) had ordered them to redo their fast. If the Prophet (peace and blessings be upon him) had ordered them to redo their fast, it would certainly have been widely reported. We have also another report confirming the same from `Umar ibn Al-Khattab: Once he had broken his fast presuming that the sun had already set, but later on the sun appeared again, whence he said, “We would not redo the fast, for we did not deliberately do anything wrong!”
Based on this and similar evidence, Imam Ibn Taymiyyah has rightly concluded that one need not redo the fast broken through such mistakes. This view seems to be more in line with the spirit of Shari`ah, which is based on ease and simplicity rather than rigidity and hardship. Allah has ordered us to pray, (Our Lord! Take us not task if we forget or unwittingly do wrong!) (Al-Baqarah: 285)