It's quite easy to build an accurate ticker, that monotonically increases at a rate that is within a few PPM of a known frequency. This is basically the "add finest cheap Casio digital watch technology to motherboard" option. If you fill your datacenter with these, they will slowly drift apart from each other and from UTC. UTC is generally what you want but then you have to account for leap seconds.
There is also huge scope for argument about what is meant by "very accurate".
The TSC is sort of supposed to fill this role but has limitations. https://lwn.net/Articles/209101/
It's quite easy to build an accurate ticker, that monotonically increases at a rate that is within a few PPM of a known frequency. This is basically the "add finest cheap Casio digital watch technology to motherboard" option. If you fill your datacenter with these, they will slowly drift apart from each other and from UTC. UTC is generally what you want but then you have to account for leap seconds.
There is also huge scope for argument about what is meant by "very accurate".