Lots of GPS clocks use poor GPS modules or some only turn on the GPS periodically to set a crystal RTC. Many GPS libraries force a 3D lat/Lon lock before passing the time/date. But time/caste dies not require GPS position lock!
I modified the Adafruit GPS lib to only provide time/date info. It does this very quickly... By not requiring a lock, I easily get GPS signal in my basement during the worst storms!
http://forum.arduino.cc/index.php?topic=199216.0
Or
http://www.hackster.io/rayburne/5-billion-arduino-gps-clock-for-25