If you want to know the entry date of a qualified plan, you simply look at the plan document and/or adoption agreement. I've seen calendar year plans that allow entry on the first July 01 or Jan 01, immediately after a participant has met the plan's eligibilty requirements. I've also seen plans that allow entry on first payroll period immediately after eligibility.

But where does one find the entry date on the Form 5305-SEP IRS form?

The 5305-SEP (in my question) requires age of at least 21 and 3 years of service.

The employee in my question began employment in year 2001 (he was age 18). He turned age 21 on Sept 12, 2004.

My question:
When does he enter the plan? Is it 01/01/04 ...or... Sept 12, 2004?

If he enters on Sept 12, 2004, then his eligible compensation would be from 09/12/04 - 12/31/04 .... right?

(His YTD gross salary in 2004 is over $ 450).

As of the first day of the plan year in which the eligibility requirements (age, service, and compensation) are satisfied, to wit, 2004.

[IRC 408(k)(2), ........ "satisfied" ... "for a year" ....]

The regulation contain an example in which an employee that turned age 21 on December 31, participated as of the first day of the calendar plan year. Compensation is not prorated.