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