Lazy Delete for Email

So I'm going through my inbox this morning (procrastinating on my nserc application, btw), and I notice I have a lot of messages that take the form "Don't forget about on ". After reading, I don't want to delete this message, because having it in my inbox serves as a nice reminder about whatever it is I'm not supposed to forget about. However, after a week my inbox is now clogged with messages about events that have passed. What I would like to be able to do is, when initially reading the message, click a button next to the Delete button, lets call it Delete On ... and then I can specify a date, so that the message will be deleted once it has become invalid.

Maybe run some kind of computational linguistic method to read the message first and propose a deletion date?