SQL doesn't use high-precision timers (too resource intensive). Inaccuracy in the order of a few ms is not a problem. Like most rounding issues, the large numbers are accurate enough and a couple ms here or there is not going to skew performance tests, because you should take multiple tests and average.
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
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