Thread from last year.
Back in 2018 I emailed NIST and inquired about time standards. Here is the response I received from one of their physicists:
If you would like a NIST-traceable standard for calibration of a battery-operated clock, there are a few alternatives you might consider. One is a clock or watch that receives WWVB. Another is an ordinary cell phone - presuming that your phone has an app that displays seconds. It’s common practice in cellular telephony to have GPS receivers at the base stations, since the base stations have to be synchronized to a microsecond or better. That’s the time that the base station communicates to the phone. GPS time is traceable to USNO, and NIST and USNO have a memorandum of agreement that our time scales are equivalent to 500 nanoseconds.