Many people acknowledge October 31 2008 as the “birth” of Bitcoin by way of the white paper. These individuals are not recognizing the birth of the first block itself, but simply the birth of the idea being made by way of a public announcement to gain critical opinion and possibly further developmental support. A few clever people took his bait.
Mow writes a good summary as to why Satoshi's idea as described on 9 pages is not a technical scope document of a major software project and this difference is helpful by providing clarity and understanding.
The actual source code for the pre-release version came out November 16 2008, and version 0.1.0 on January 9th, 2009, after the Genisis Block was created. Satoshi himself stated his code began development at least 1.5 years prior.