Namaz Before It's Time
Salah is not dependent upon hearing of an Azan, it is dependent upon its due time. So if the time of Isha came, then one can offer Salah irrespective of the fact whether one has heard Azan or not. If the time is still not due and one has mistakenly prayed that Salah, then upon realization, one should offer Salah on its time.
So the due time is basically the time which is on Google. I mean when you search for namaz times it tells you based on the university of Islamic Sciences Karachi. So this means that we should consider this time right?
Yes you’re right and honestly I don’t know myself that when did people start looking at clocks but still I have no idea about the times(by looking out) so for accuracy I consider the clock.
And with respect to when Isha begins, it is at the disappearance of the red twilight (shafaq ahmar) on the horizon according to the jumhur, though waiting till the disappearance of the white twilight (shafaq abyad) is preferable.