Period of Manufacture

My apologies for the repetitiveness of this question; I have reviewed the comments posted to date and it is still not clear to me: I am using LEED CI V.1, 2009, and am struggling with the period of manufacture dates. I have collected the greenguard certificates, and in the "period of manufacture" columns have entered the dates for certification directly from the GreenGuard Certificate of Compliance; is this acceptable? I have made sure to obtain certificates prior to the start of the project (this goes back to 2008, so I don't know how to go about tracking down the dates of manufacture now?). Any advise would be much appreciated!