When One Should Stop Eating Before Fajr - (Imsak)
In the Maliki madhab it is recommended to stop eating a short time before the entrance of the dawn (fajr) time. This is based on the action of the Messenger of Allah (peace and blessings be upon him) who would stop eating before dawn the amount of time it would take to read 50 verses.
If a person were to eat right up until the entrance of dawn, the fast would technically be correct but they would have left the recommended Sunna. Once should exercise caution though in eating close to the dawn time. This caution is especially needed when you are not sure about the entrance of the time, such as when it is cloudy or you are in a valley.
And Allah knows best.