I think the US does at least a decent job of providing accurate time at: https://www.time.gov.
That said, I needed a challenge this afternoon.
How about this (it seems to match https://www.time.gov):
First cell:
now = DateString[Now, "ISODateTime"];
Dynamic[now]
Second cell:
i = 1;
While [i < 10000000, i++; now = DateString[Now, "ISODateTime"]]
Though you will have to force the engine to stop.
