Date manipulation in PHP

I was sitting writing a kind of helpdesk call management program in PHP and found that I needed to work out how much time had elapsed since somebody had last bothered to contact the client about their problem. In the past, when I used to use ASP, the solution was simple - ASP has a command called DateDiff which takes two dates and can tell you how many seconds has passed between them, how many days, months and so on. After scouring the PHP manual for a while I decided that PHP has no such beast. I then wondered how many other people there were out there like me, and thus this tutorial