The thing is, is that the phone time isn't always accurate either. Some cell networks do not provide the NITZ time, and the phone's internal time can drift.
Starting with Android version 3.0 ie Honeycomb, there is a function which automatically updates the phones time using NTP. It checks in every 24 hours to NTP, and updates the phones time if the drift is more than 5 seconds.
But still your phone could be 5s out. What if you want better?
Handily there is an App for that: ClockSync <https://market.android.com/details?id=ru.org.amip.ClockSync>
ClockSync shows system ie phone time and Atomic time ie NTP time, so you can set your camera to Atomic time and be super accurate with your GPS tagging.