coopster - 9:25 pm on Jan 29, 2008 (gmt 0) I find it easiest to push the dates into an array as unix timestamps and grab the max from the array.
Don't forget to run each of your included libraries as well as any database table timestamp entries through your date routines to grab and store the latest date for comparison. For example, if you have a content system that is including a header, a footer, another included content file from the filesystem or perhaps even an entry from a database table (hopefully your content entry in the database tables(s) are being given last modified timestamps!) you will want to check the latest date entry and use that in your If-Last_Modified comparison.
I find it easiest to push the dates into an array as unix timestamps and grab the max from the array.