Ok, you'll have to explain this to me, unless it's humor :)
If the server sends the current time with the header, and page is set to expire at that exact time, how does the client's clock even come into it?
-- man with no legs, inc.
The server says "this expires at 3:27". How does the client machine know when it's 3:27? It checks its own clock. That clock can be off. Way off. This kind of problem happens any time you have two machines talking about things happening at exact times but their clocks are not synchronized.