difference between two times? Date_diff and DateTime::diff

Hello, My journey of learning PHP is going well, so i've decided to make a small program which works out the difference between two times and records the data in a text file. I've searched the PHP manual for functions which can help me out, and I discovered the function Date_diff ( http://uk3.php.net/manual/en/function.date-diff.php )and the class DateTime::diff ( http://uk3.php.net/manual/en/datetime.diff.php ) My question is, how on earth do I use these functions ? I really don't understand the manual documentation. I've just moved onto the subject of classes and so I'm fairly new to the concept, although I am following it well. If someone could explain to me how to use ether of these ( Date_diff and DateTime::diff ) I would be VERY grateful. Thank you so much! Matt