Question: The mu’ath-thin pronounced Athaan (call to Salaah) for Maghrib (Sunset) Prayer at 5:37 P.M. And Madfa` Al-Iftaar (the canon signaling breaking the fast time) was heard at the same time, followed by other mu’ath-thins pronouncing Athaan. The people started breaking their fast. Those mu’ath-thins thought that the time of Maghrib had already started and the sun had set, because it was cloudy, although the official time of Maghrib as set in the calendar and determined by the Department of Astronomy in the University of King Su`ood was at 5:45 P.M. Then Madfa` Al-Iftaar was heard again at this time, and other mu’ath-thins pronounced Athaan, and some people started breaking their fast at that time.
Your Eminence Shaykh, people are confused; some of them believe fast of that day was nullified and must be made up for due to such negligence of determining the right time, since the clock was available and they could know the exact time. Others said that we do not have to make up for this day, and the one who carries this sin and should make up for the day is the one who caused this problem. Therefore, we request Your Eminence to quickly resolve this issue, so that this problem will be settled. May Allaah grant you success and guide your steps on the Straight Path.
Answer: If the case is as you mentioned, then they should make up for that day, because they broke their fast before the due time of breaking fast and due to their negligence in determining the right time... read more here.