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. Our timetable is based upon the depression of the sun at 17.5 degrees.
Many other centers follow the times formulated by London Central Mosque and have the starting time suhoor later than ours. We no longer follow these times because the period of persistent twilight during Ramadan no longer exists. A more full and complete explanation can be found at moonsighting.co.uk.