Briefly late deposit of deferrals for several years

[Guest Penelope]

Client is a non-profit with an ERISA-covered 403(b) plan. They believed (based on a seminar the finance officer attended) that they had until the 15th day following the end of the month deferrals were withheld to deposit the deferrals with the insurance company, and they've been doing that for quite a few years. Now they are facing the compliance question about late deposits on the 5500. They've become aware that the 15-day limit was not a safe harbor.

The question on the 5500 concerns only 2009 deposits, but presumably if they answer yes, the DOL is likely to take an interest in earlier years.

Has anyone had any experience dealing with the DOL on a correction for this? Would it be reasonable to take 1996 as the date when this violation began?