Very good question!
1. The I-485 receipt notice contains two dates:
Receipt Date (RD) - the date when the package reached the TSC mail room.
Notice Date (ND) - the date when the information was entered in the TSC system and the I-485 receipt notice notice was issued.
2. The standard online message shows the following:
"Your I485 Application to Register Permanent Residence or to Adjust Status was received on NOTICE DATE. We mailed you a receipt with information about processing. It is taking between 990 and 999 days for us to process this kind of case. We will mail you a decision as soon as processing is complete."
3. The comments on the USCIS Processing Times page are:
"The date shown on this chart corresponds to the date the case was received by the Service Center."
From the above it looks like they are using the NOTICE DATE as the date when the petition was actually received by them, which is very unfortunate for the May, June 2001 filers.
However one might argue that since they talk about "the date the case was received by the Service Center" as the date published on their processing times report, it should be compared to the Receipt Date on the I-485 receipt notice.
Here is a tip: If the online message has changed from the "Received" to "FP completed" mode the 1-800 customer service folks have no way of knowing the RD vs ND and if the official date has passed the RD one might ask for expedite processing and get the case looked at.