What should be done by one who is uncertain about the time of the true dawn and leads people in prayer as an Imam before being certain of its entry?

Calendars vary in determining the time of Fajr prayer, and therefore, one should only follow the most reliable and accurate among them. There is no harm in following the Umm Al-Qura calendar, even though Sheikh Ibn Uthaymeen believed that it sets the Fajr time too early. Prayer is not permissible unless one is certain or has a strong presumption that the time has begun. It is preferable to go to a mosque where it is highly probable that the prayer will commence at the correct time, and one should not pray behind someone who is believed to start the prayer before its appointed time.