Pulsars are a secure time source

"Nature provides clocks that are oblivious to the malicious intentions of any outside parties. In the case of a remote high-energy system such as a pulsar, this means anybody. Pulsars are many orders of magnitude more accurate than random delays that face attackers on the Internet. If critical Internet servers were synchronized to natural clocks in a secure and timely fashion, they would be immune to attacks that relied on uncertainties in timing."