If you're talking about time display -- like you would see with, oh, say, xclock or the desk top clock display, about the best you can do is use NTP to synchronize your system clock to time serves that are referenced to atomic clocks around the world. When NTP synchronizes, you're pretty much dead on the correct time. If you need higher accuracy (and we're talking about picking nits here), you can invest in a GPS clock which may give you a millisecond better accuracy than NTP, but you're talking about hundreds of dollars to obtain that. You might want to try NTP (which should be on your system already) first and see if that does what you want.
Hope this helps some.