This is more a Linux task than a PHP one, but for those needing to change their server date/time, it can be done with the linux DATE command as follows:
date - s `+ 1500`
This might require root or sudo privileges. In that case the command would be:
sudo date - s `+ 1500`
On a PHP note, I broke down ways to work with the PHP date() function in my php-scripts diary on 12/17/1999. It doesn’t seem like that long ago, but it was nearly six years ago!