Just like my VAHR, my wife's FR645 didn't need a GPS-fix for the exact time. It simply got it by syncing with the phone/GC. So I guess the newer devices (since VAHR, at least), don't need GPS for the time. Her old FR235 and my fenix 3 HR on the other hand, needed a GPS-fix.
Still, if the FR645 refuses to get the time from syncing with your phone, you will have to get a GPS fix. If this won't work neither, I'd recommend a hard reset, followed by a sync with GE (NOT GC) and a follow-up GPS-fix.
- From the watch face, hold UP.
- Select Settings > System > Time > Sync With GPS.
- Wait while the device locates satellites (Acquiring Satellite Signals).