Prior to the widespread use two way satellite time transfer (and secondarily GPS) for this purpose ( https://www.microsemi.com/document-portal/doc_view/133433-w

Prior to the widespread use two way satellite time transfer (and secondarily GPS) for this purpose ( https://www.microsemi.com/document-portal/doc_view/133433-working-application-of-twstt-for-high-precision-remote-synchronization https://www.microsemi.com/document-portal/doc_view/133433-wo... )-- taking an atomic clock on a road trip was the standard way to obtain the second: https://ntrs.nasa.gov/api/citations/19820012622/downloads/19820012622.pdf https://ntrs.nasa.gov/api/citations/19820012622/downloads/19... There were even more portable units built specifically for this time transfer purpose. I don't think NIST has a regular service for this anymore, but since they do offer a host of calibration and verification services, I'd be surprised if you couldn't road trip a clock to obtain the second... at least for a price. I've seen some publications on transportable optical clocks in recent years, which I assume are motivated by validating newer higher precision TWSTT and fiber time transfer techniques.