When Did the Recession Begin?

As noted in the post below this one, the NBER has dated the beginning of the recession as December 2007. Brad DeLong says that "I think that this was the right way to call it..." However a colleague, Jeremy Piger, has a recession dating model that indicates it wasn't exactly clear when the recession began. His recession probabilities are:

200705	1.2%
200706	1.4%
200707	1.8%
200708	2.9%
200709	4.6%
200710	7.3%
200711	10.4%
200712	17.1%
200801	23.6%
200802	33.1%
200803	37.4%
200804	42.7%
200805	47.7%
200806	54.9%
200807	66.0%
200808	96.2%
200809	99.2%

According to these numbers, one could reasonably put the peak anywhere from October 2007 to May of 2008. Here's a graph of the recession probabilities from June 1967 through September 2008: