Accurate GPS-based timestamp facility for Gran Sasso National Laboratory

A new system to assign accurate timestamps to events recorded by experiments running underground has been designed, installed and tested at INFN Gran Sasso National Laboratory, Italy. This facility is based on a Master unit installed on surface and receiving time information from a GPS receiver, and Slave units placed underground which get data packet from the Master via optical fiber and assign the timestamps. The system is able to provide a time accuracy of 15 ns (1 $\sigma$) and precise reference frequencies to the experiments. It is now part of the infrastructure of the Laboratory for all the experimental activities which need accurate timestamps.