Are fasts broken if you don't follow imsak timing?

I just found out that my previous fasts might not be valid but please help me confirm if it's true or not. Whenever I want to fast, I have suhoor until fajr time. My friend tells me that's wrong and my fast is broken because I didn't follow the imsak time which is approximately 10 minutes because fajr time. For example: If fajr time is 6:10, then imsak time is 6:00. So I should stop eating and drinking at imsak time. Is this true? If it is true and I didn't know about it, are my previous fasts valid?