To set the date, type date MM:DD:YYYY (American style date) where MM is month, DD is the number of days within the month and YYYY is the year.

For example to set the date to the 1st January 2000 you would type:

date 01:01:2000

To set the time (where the -s option is to set a new time), type:

date -s hh:mm:ss