I have a binder of basically all regular promos (things fairly easy to obtain) since
Time Spiral, when I started playing.
The prerelease date stamp was gold on everything, and starting at Shards, the release card also had a gold date stamp.
At M12, neither promo has any stamp at all, and the same thing with Innistrad. Dark Ascension brought back the stamps, and it looks like the prerelease card has a silver date stamp (over a gold-yellow background...), and the release one has a gold. Then, they ditched the dates again for Avacyn and M13. RTR you've mentioned, and the release card is gold again.