How MUCH drift have you got ? What is the operating temperature range ? We did this problem to death a few months ago, so search the site for the answers.

Failing that, Charles points out a very very accurate time source. In Europe there is a German transmitter, and in the UK there is MSF Rugby. The time signal is usually good to around 800 km from the transmitter. I don't know about the rest of the world.

Of course there is always GPS ! In extremis, THAT is good to < 30nsec RMS ANYWHERE

Steve