I always use my computer's clock for that, in my time it begins 4:31:00, i couldn't notice it and i drank the water in my mouth at 4:31:07, is my fasting valid?
It sounds like you knew you were going to be fasting, and you did not realize the fast had started. This sounds like it is squarely in the category of forgetful breaking of the fast and therefore your fast is not invalidated by that action.
There's a pretty solid breakdown of the rules here at The Fiqh of Fasting and here it is related that
The Prophet (Allah bless him and give him peace) said, “Whoever forgets that he is fasting and eats or drinks, then he still completes his fast. It is only Allah who fed him and gave him drink.” [Bukhari]
