I cannot find what is considered a new account under Reg E (error resolution) as clear as it is found under Reg CC. Do we get to make up our own definition via procedure/policy?

An account opened on or after July 1, 2010 is a new account for Reg E purposes.
12 CFR 1005.17

Thanks so much for your feedback. I am wondering about how an account is defined as "new" under Reg E error resolution procedures in which permits us with 20 business days to investigate an error instead of 10. § 1005.11(c)(3)(i)
I cannot see where it is defined, so I am wondering if I define it myself, or if I am missing it somewhere in the commentary/guidance?

Sorry, I pulled the wrong reg. section. Based on the section below, I would say an account is "new" for 30 days after the first deposit. (Quotes are from 12 CFR 1005.11(c)(3)(i) and (ii)).

(i) The applicable time is 20 business days in place of 10 business days under paragraphs (c)(1) and (2) of this section if the notice of error involves an electronic fund transfer to or from the account within 30 days after the first deposit to the account was made.

(ii) The applicable time is 90 days in place of 45 days under paragraph (c)(2) of this section, for completing an investigation, if a notice of error involves an electronic fund transfer that:

(A) Was not initiated within a state;

(B) Resulted from a point-of-sale debit card transaction; or

(C) Occurred within 30 days after the first deposit to the account was made.

Thank you. I don't know why I had such a hard time seeing it - It was right there the whole time!

It would be helpful if definitions were actually in the section of the regulation called "Definitions."

There is no definition there because the rule is based on the number of days from the deposit opening the account, not on whether it meets the definition of the words "new account." In fact, that phrase appears nowhere in section 1005.11 or its Official Interpretations.