Comparing two dates.. Which is the older one?

Greetings. (yes.. I am active) When a user loggs on, I pass the string $loggtime, that is written to a file. Whenever a this user refreshes any of the pages, certain tasks are performed, and a file's contents is checked, (which contents is a single line of time). If the $loggtime is newer, nothing would happen, but if the $loggtime is older than what is written in the file, something should happen. Any ideas of how I can compare two dates? - Richard ps: to all who wonders, I am writing a really small community. Now, this question is related whenever a new message is sent.. The message is written to a file, and I check that file everytime a site is refreshed. The file is arrayed with "|" as a separator. :Ds