What is with noobs coming in and asking for features that have already been discussed before many many times? Please, do one courtesy search, and you would find many discussions and reasons as to why they have not been implemented. Here’s a summary anyways.
The general thought is that conversations here are not meant to time out, thus there is no real point in having the datetime, because it would take up some space in the really clean cut comment format (and hiding it under something wouldn’t stop questions like this from appearing, not to mention such “mystery meat” web design has many problems). I heartily disagree with your opinion regarding conversation “expiration dates” @Kelly27, I certainly have never seen them on this site nor in real life. Another issue about posting a date/timestamp is making it work correctly with all the timezones, not to mention there may be confusion issues if people choose to quote other using the date/timestamp (I could quote you and the timestamp could be from your future if we were in different timezones). A third consideration is that a timestamp would make this place feel more like a forum instead of a general archive of knowledge and experiences.
That said, they have stated they may look into possible ways of integrating it, but that they hesitate to add elements that would repeat constantly.