It was early-mid September 1997. Strangely I'm having trouble finding the exact date online, and the original readme file's last updated date was in August of that year, but I know for sure it was September since Total Annihilation came out about 2-3 weeks later in late September.
Edit: The latest modified date on the original CD was 8-11-1997. That would be when the game went gold though, so it'd actually had hit the shelves a few weeks later. Still can't dig up the actual date for that.
Edit 2: http://forum.outpost2.net/index.php/topic,3917.0.html
According to this, we determined it was September 1 1997 based on a Sierra press release that I decided not to actually link to back then, which I can't seem to find. Tried digging through sierra.com on archive.org a bit.