The Official Website of Grand Ayatollah Makarem Shirazi

if one is not certain whether the time of breaking the fast has arrived, they must not break their fast, and if they do, they must both make up for the missed fast and give kaffārah for breaking the fast intentionally. But as for Fajr-Adhān, if one doubts whether or not the Adhān has been announced, they can do things that invalidate fasting, and it is not incumbent on them to investigate. However, if one commits one of the fast invalidators and then finds out that Fajr had already set out, they must make up for the missed fast. Looking at one's watch or clock is also considered to be an investigation, provided that they are accurate. Eating Saḥūr is recommended, but based on obligatory precaution, the one observing fast should abstain from eating and drinking before the Fajr-Adhān is announced. However, the time of maghrib-Adhān is considered the end of fasting and one can break their fast at the moment Adhān is being announced. It is recommended for the fasting person to perform their prayer before breaking the fast. However, if there is no presence of the heart or if someone is waiting for them, they had better break their fast first, but they should try to perform the prayers within their privileged time. In circumstances in which one lives in some region of the earth where there is an extreme variation in daylight hours and nights. For example, days could extend to more than 18 hours, and one observing fast is not able to fast for such long hours. They can fast according to the horizon of areas with moderate hours, such as the city of Tehran. Meaning, one can start fasting for 15 hours from the time of Fajr-Adhān if the fasts in Tehran last for 15 hours and then they can break the fast. Nevertheless, if one has the ability to fast completely in such regions, then they should act accordingly.