There is certainly hardware available that can do better than this, but the parent asked about cell sites in specific (and cell sites are one of the top users of the Thunderbolt). You can get GPS hardware based on atomic references, for example, that will do better over these kinds of timespans.

(You really need GPS or some other precision time transfer mechanism for long-term accuracy, though ... there's no getting past that.)

Well...if you're in that position, you probably have money to spend on failover config. Pretty accurate hardware clock modules were available even before GPS. Not cheap, not simple, but reliable - if you have the need and the money.