Fasting begins from the first light of the day and this occurs in the early hours of the morning. To make it easier, many mosques & Islamic Centres put together timetables which have the starting times. There may be some differences in these timetables based upon the method they used to extract the times.
Traditionally, most Muslims follow the observations made by Muslim World League or University of Islamic Sciences, Karachi who have agreed with classical scholars such as al-Biruni that we the correct times are based on about 17.5 or 18 degrees.
In the UK, we suffer from persistent twilight in summer months for which a new standard was established in 2012 by London Central Mosque and have the starting time suhoor later.
Our timetable is based on 17.5 degrees which is a traditional standard as we feel it is better to be cautious and safeguard fasting. The difference in the earluy start is less than 15 minutes. Our Ramadan calendar follows the times published by al Muwaqqit.