To be useful, any atomic clocks you operate must be synchronized to a Stratum Zero time source, such as GPS. Such clocks are useful when you need exceptional accuracy, such as for Building Integrated Timing Service (BITS), but unless they’re synchronized you can’t coordinate time-sensitive activities such as digital certificate validation with anyone else on the Internet. 

From https://www.gps.gov/applications/timing/

Each GPS satellite contains multiple atomic clocks that contribute very precise time data to the GPS signals. GPS receivers decode these signals, effectively synchronizing each receiver to the atomic clocks. This enables users to determine the time to within 100 billionths of a second, without the cost of owning and operating atomic clocks.

Precise time is crucial to a variety of economic activities around the world. Communication systems, electrical power grids, and financial networks all rely on precision timing for synchronization and operational efficiency. The free availability of GPS time has enabled cost savings for companies that depend on precise time and has led to significant advances in capability.

Forrest Christian (List Account) wrote:

In the middle tends to be a more moderate solution which involves a mix of

time transmission methods from a variety of geographically and/or network

diverse sources.  Taking time from the public trusted ntp servers and

adding lower cost GPS receivers at diverse points in your network seems

like a good compromise in the middle.  That way,  only coordinated attacks

will be successful.

Instead, just rely on atomic clocks operated by you. They are not
so expensive (several thousand dollars) and should be accurate
enough without adjustment for hundreds of years. There can be no
coordinated attacks. They may be remotely accessed through
secured NTP.

                   Masataka Ohta