Helllllllo! Well excuse me. We're in the citizenship discussion forum here, not the permanent residency one! In this case, the priority date IS when the pertinent USCIS service center receives the N-400. It will be on the N-400 receipt notice, which usually comes 7-10 days after the PD.
And -- for what it's worth -- some of us didn't have to go through LC during the permanent residency process so our priority date WAS when the I-140 was received by the USCIS (for me, INS at that time). I was in the EB-1 preference category for my I-140.