See NTPServers.
GPS time signals have an accuracy of ±10ns, making them the best option for most applications. The planned establishment of Galileo by the European Union and Compass by China will provide additional sources of time for compatible receivers.
It's possible to 
sync a computer directly to the Pulse Per Second (PPS) output of a suitable GPS device using a serial or parallel port. For better accuracy, there are PCI cards available with PPS inputs and/or built in GPS receivers. There are a variety of dedicated NTP servers available, some with internal oscillators that can maintain a stable clock if the satellite signal is lost.
If you require better accuracy than GPS can provide, OmniSTAR provides GPS and GLONASS correction data via a worldwide network of geostationary satellites and reference stations.
It might be possible to receive the 5, 10 and 15 MHz signals broadcast from NIST's WWVH facility in Hawaii. WWVH continuously broadcasts a time code on a 100-Hz subcarrier. The time code presents UTC(NIST) information in BCD at a rate of 1 pulse per second. (See 
broadcast format.)
Radio New Zealand broadcasts UTC(MSL) time signals on many of it's stations every hour. They consist of six “pips” of 1000 Hz tone, at one second intervals, the beginning of each pip marking the exact second. When a pip marks the exact hour, its length is doubled. When a leap second is inserted seven pips are broadcast.
Freeview transmits local time information using DVB-T at least once every 15 seconds with an accuracy of ±2s.
2 pages link to TimeSources:
