Does that matter? When one signs and dates it, it only asserts to the time when the signature was put. It need not be the same as the date it was filed. When one goes through the attorney, the signatures may be way earlier than the filing date. However, the date at which USCIS receives cannot be earlier than the 90-day limit because it fails that test.
And if I am not mistaken, there is absolutely no statement about signature date. My $0.02.
Poongunranar - You have raised an interesting point. For those who might remember, my N400 application was simply returned unopened after reaching lockbox in phoenix. Date of signature on N400 and check were both 6/19/2010 (88 days before 5 year). I received it a week later in mail and decided to simply resend original paper work by FedEX. When I received NOA, the PD is 6/29. Which was also the date FedEX delivered it. So it looks like it's only the date when N400 was received and and generated PD are contenders in this equation. Not the date when it was signed.