Current plan document language will probably address whether you use the plan's entry date or the statutory semi-annual entry dates. Our PPA VS document says you use semi-annual entry dates.
If you look at an older document, it likely won't address it. The IRS speaker's 2006 comment was basically that you had to use the plan's entry date, unless the plan language said you use something else. The ASPPA panelist's reply was that if we had known we needed to put that in our documents, it would be in there.