But how does it determine positions from just time? I may seem stupid but I can't make the connection.Maybe this should be split off into a different thread.
Differential GPS involves comparing times of clocks on the ground and in satellites. Essentially, GPS devices at known locations on the Earth's surface are treated as satellites with orbital radii given by the their distances from an Earth-centered frame, and with orbital periods equal to a day. A differential receiver uses both real satellites and Earth-bound "satellites" to determine its position, so there are GR timing differences between two "satellites" due to their much different r coordinates.