My understanding is - but no guarantee - that when we put a date in a list, we just put a reference to the variable which continue to live.
When we insert a string we really insert the group of characters as a dead item.
A few weeks ago, in another thread, Has, in one of his complaints about appleScript, mentioned that dates were a problem because the were mutable.
That puzzled me, but I think this may be what he meant.