Google Groups has an archive of many newsgroup posts dating very far back. If you search these archives you will find posts suggesting that the game was released after August and as far back as September 26th. One may think that these newsgroup posts are due to simple distribution issues. Which may be a fair point, however a press release was recently discovered and given to us by an anonymous a former Westwood employee. This press release, presumably released to game magazines and similar publications, announces that Westwood Studios has released "Command & Conquer" and that 200,000 units of the game had been shipped. The date of the document is September 26th, 1995. Giving a lot more credibility that the game was indeed released in late September.
Adding to the legitimacy of the press releases origins is that the document was saved in Microsoft Word 6.0, the address on the footer is correct and that it was last saved on September 26th 1995.
This document together with the newsgroup posts is why Command & Conquer is listed as being released on September 26th on this website.