Cell tower time. Modern cell communications use time-slot multiplexing with transmission slots in the scale of a millisecond and need the beginning/end of trans

Cell tower time. Modern cell communications use time-slot multiplexing with transmission slots in the scale of a millisecond and need the beginning/end of transmission to have microsecond-scale accuracy so that you don't miss stuff and don't step on the toes of the previous or next transmission, they need to adjust for light speed propagation, etc. So as a byproduct of needing to continuously sync time with the cell tower in order to function properly, your phone has quite accurate time; your computer might easily be half a second off of 'true' time, but your phone (at least on the broadband chip level - the main OS can not care) can't be even a millisecond off.