Login as root or su to become root and enter the root password.
# date hh:mm:ss
Explanation: hh means hours, mm means minutes and ss means seconds.
Or to adjust date and time, then use this scheme:
mmttssmmjjjj
this means: MonthDayHourMinutesYear
Example for 01.02.2008, 12:30, input:
# date 020112302008
At Debian Wheezy it worked with
# date --set 2013-05-07
# date --set 15:00:00