So far SmartPlaylist could only support Release Dates after 2001, because Apple's programming framework only provided NULL/nil/nada/nothing for older dates (there was no life before the iPod, was there?). Now I have found a second way to also extract older Release Dates.
As this takes a bit of time, searching or sorting by Release Date is a bit slow at first. SmartPlaylist puts the extracted values in a cash to speed up sorting and searching the next time. This cache is emptied with "Empty cache after sync" (see other posts).