The Official Website of Grand Ayatollah Makarem Shirazi

If one owes a vow fast, they cannot observe it while traveling. And even if they make a vow to observe fasting on a journey, or make a vow to fast whether on a journey or at home, they cannot fast on a journey, and this kind of vow is even problematic. Even if one makes a vow to fast on a certain day such as mid-Sha'ban, based on obligatory precaution, they should not travel on that day. And if they do so, if possible, they should stay for 10 days in a place where they can fulfill their vow. In addition to obligatory fasts, all of the recommended fasts are invalid to observe while traveling except for the Hajat fasts, which are specific to Medina, and one can observe them even when they stay less than 10 days in Medina. Hajat fasts must be observed in three consecutive days, and the obligatory precaution is for those three days to be Wednesday, Thursday, and Friday. Thanks to technological advances, many trips are taken by airplane. Therefore, transportation may take place at a higher speed than the change in local time zone. In that case, if one travels by air to the western regions after dohr, yet they arrive in the destination on the morning time on the same day, they must abstain from things that invalidate fasting until the Maghrib Adhān according to the time in the destination. Also, if one travels by air in the morning and arrives in their hometown before the Fajr Adhān, they can eat and drink again until the Fajr Adhān and then observe their fast. If on the last day of Ramadan, one travels to a place where Ramadan has not finished yet or Eid al-Fiṭr has arrived earlier, they should act according to the local time, even if that means they are going to fast more than 30 days or fewer than 29 days in total.