Very neat (and easy to remember) website. I will definitely be using this, thanks.
As far as accuracy, for a while I ran a GPS-based stratum-1 time server, but I took that apart to use the components for some other experiments and never put it back together... Now I just run a regular old NTP server on my LAN, which gets its time from official time servers at NRC in Canada and NIST in the US. That system currently thinks its about 0.5ms off, and a desktop machine on the network says:
Your time is exact!
The difference from Time.is was -0.003 seconds (±0.025 seconds).
My Windows machines sync to the same server, but the SNTP client in Windows is pathetic and can barely get within a second even just after syncing. You need to run a proper NTP implementation to get decent time on a Windows machine.