Yeah, I also think that in the early days no one care much about release date but games had to be manufactured and shiped to stores and someone should take a note damnit! :) It's funny that I can find premiere date for a silent movie from 1920 and the industry that is alive for about 30 years don't care about its legacy.
I was trying to use CGW but as -->superstande said even there I found nothing.
And yes I think some of GOG/STEAM dates are made up and no secret knowledge is involved.
It's really not a big dill BUT when one game premiere in January and other in December you can clearly see who copycat who :)
EDIT:
rgnrk: I've also notice than sometimes when there's only the month and year of the release date (in the game credits, or the release information in mobygames or the wikipedia, GOG assumes the first day of the month as the release date for its gamecard.
Yup. I think that may be the case.