small business size eligibility "date of initial offer" - proposal submission date or proposal due date?

13 CFR 121.404. There might be a FAR reference but i didn't look for it. Always can go to the SBA and let them make that determination. I'm not certain.

Yes, I see size is determined "as of the date the concern submits a written self-certification that it is small to the procuring activity as part of its initial offer or response which includes price." - So that is the date the paperwork (133 or whatever) is signed or the date of an email or portal transmission, not the date the proposal is due?  It's an interesting outcome in cases where there are a lot of proposal due date extensions.

The date the paperwork was signed and the date the initial proposal is due is not, in my opinion, relevant to your quoted sentence. You may wish to do some FAR/CFR research as to the term "submits" and verify that your company "submitted" a written self-certification and price on or before September 2nd. There may be situations where the time of day of the submission is important. For example, if you successfully "submitted" on the West coast at 9:01 pm, and the requirement is interpreted by East coast time, it was September 3rd on the East coast at the time of submission and your firm does not qualify.