Servers with large uptime, might gather quite a few seconds of drift in their clocks. To synchronize the server's time with an external source, simple run the following:
sudo apt-get install ntp ntpdate
sudo ntpdate pool.ntp.org ntp.ubuntu.com
If you get an error while about the port being in use, then you might try killing the previous running ntp process and trying again.
It would also be a good idea to add the second line to your cron tab as a daily run.