As an example, to set the current time and date to July 31, 11:16pm, type "date 07312316" (note that the time is given in 24 hour notation).
If you wanted to change the year as well, you could type "date 073123161998".
To set the seconds as well, type "date 07312316.30" or "date 073123161998.30".
To see what Linux thinks the current local time is, run date with no arguments
More information can be found here:
http://www.linuxsa.org.au/tips/time.html
