Form 5330

Plan had late deferrals that were all funded during the plan year.

The recordkeeper posts the contribution and then a few days later the income is posted.  Recordkeeper requires the Sponsor to approve the ACH for the income.

Regarding the Form 5330

Schedule C #5  - date of correction

is this the date the file was sent to the recordkeeper?

or

the date the deferrals were posted

or

the date the lost income was actually posted to the account

for the 5330, my guess is it's when you've fixed the prohibited transaction, so (2)

for the 5500/DOL and fiduciary liability, I'd suspect it's (3)

The Form 5330 Instructions for Schedule C line 5 include this:

“For purposes of section 4975(d)(23), the term ‘correct’ means to:
Undo the transaction to the extent possible and in all cases to make good to the plan or affected account any losses resulting from the transaction, and
Restore to the plan or affected account any profits made through the use of assets of the plan.” https://www.irs.gov/pub/irs-pdf/i5330.pdf

I’d say the prohibited transaction is not corrected until the plan’s trust has not only the late participant contributions but also the interest or investment gains the plan would have obtained had the contributions been promptly invested or, if greater, the interest or investment gains the employer made by having the plan’s money.

As far as when corrected I agree with the posters.  However, from your facts a more basic question ... why do you have to complete Schedule C line 5?  Maybe I am missing something but I thought you only completed line 5 when there are multiple disqualified persons involved.  Here, it seems that only the company is involved so only Schedule C lines 1-4 should be completed.

Just my thoughts so DO NOT construe my ramblings as advice

I wasn’t assuming a need to complete Form 5330 Schedule C line 5. Rather, noting only that the quoted text from the Instructions might be a useful or helpful way to think about when a prohibited transaction is corrected.

Peter, I agreed with your response.  The OP asks about "Sch C #5 date of correction"....