Only a Caesium atomic clock is the most accurate timing one can ever have. Of course, nobody has the money to get an atomic clock for oneself. So the best is to locate a public NTP server in your region that is driven by an atomic clock. Your system time is synced with the server in regular intervals, which keeps your system clock reasonably close to that of the NTP server most of the time.

Your government or a university is likely to run a public NTP server, by the way. You can also find some NTP server lists by doing a google search, like this page:

http://www.eecis.udel.edu/~mills/ntp/clock1a.html