A public service of
Reptilian Associates.
The clock you see is derived from the official US time
standard, every time reference in the US is also derived from it. The
only significant accuracy loss is due to the propagation delay between
when the USNO server gets your request for the time and when it returns
it, this should be under a second.
This clock returns a static version current time, to get an update,
reload the page.
You can see a running clock here.
For the best time accuracy, you can use utilities like Public Domain NIST Software for Windows 9.x/NT to automatically access the NIST server, convert to your local time, check and correct your PC clock. NTP does this job for *nix environments. Mac setup of (I assume) NTP is also available on this page. Information on setting NTP for Linux and OpenBSD.