Need cite for the rule that service earned while a member of an ineligible class not excluded.

[R. Butler]

Employee is a member of an ineligible class meets eligibility requirements other than that. The employee subsequently becomes member of an eligible class. I know the employee is immediately elgibile, but I need the cite that says you don't disregard service while a member of an ineligible class. I've had a brain freeze & can't find that cite on my own.

Thanks in advance for any guidance

[Harwood]

410(a)(5)(A) ?