وعليكم السلام ورحمة الله وبركاته Firstly , you should know that it is not easy to say that calendar is wrong for the whole year unless if you are following the correct method of calculating the time according to Shariah for the whole year . Because sometimes the calendar can be right too . Secondly , for those who are unable to check the time it is allowed for them to follow the Muaddin of their place . If the time in which they prayed was incorrect then the responsibility falls only on the Muaddin because لا يكلف الله نفسا إلا وسعها Allah does not burden any soul except that which it can bear . Thirdly , for those who are able to check the time and they recognise that the time in the calendar is a mistake , they are suppose to pray with the congregation as Nafil prayer . After that they should also pray again in the correct time . So they don't neglect the congregations . This is the fatwa of a lot of Ulama including Shayk Albani , Shayk Muqbil and others . Note ( 1 ) : Normally if the Adhaan of the Fajr is given in a wrong time , then the Adhaan of the Maghrib will be wrong too especially if they are following the calendar Note ( 2 ) : Don't ever accuse the validity of the prayer of the people who pray in those Masajid because sure they have a reasonable excuse. And Allah عز وجل knows best .