You can set date and time of a linux server with:

date +%Y%m%d -s "20131228"

which sets the date to the 28th of December in 2013.

date +%T -s "20:15:45"

which sets the time to a quarter past 8 PM (and 45 seconds).

If you set the system datetime, you can also set the hardware clock:

hwclock --systohc --localtime