If you use multiple cameras like we do at weddings ( 4) you need all the clocks in sync so that when you put it all into lightroom it will shuffle them all into order.
I use a Genis GPS unit ..about $50...which as two cables ,one 10 pin and a flat one for the lesser cameras ( ha ha )
Plug it in and just leave it ...NO ....plug it in and then go to the time zone menu and re set the year only to say 2014. Within seconds it will jump to the right year and time. When its only making corrections of a couple of minutes it seems to take ages on occasions.