when is a deferral considered late for for 5500 purposes for an owner

[efwrpa]

An LLC partnership filed it's company taxes by 4/15/2017.  One of the partners extended his personal return until 9/15/2017.  Originally, when we were given the census information for 2016, the owner stated he would have deferral of $18,000 for 2016.  However, we noticed he hasn't yet deposited the contribution.  When questioned, owner first said he wouldn't make the contribution, but has now decided he will reflect deferral of $18,000 on his 2016 personal return, and will deposit it by 9/15/2017.  Obviously, the deferral is considered late, but should it be declared as a late deposit on the 2016 5500?  I am of 2 minds: 1) yes, it should be on the 2016 5500, because it is attributable to the 2016 plan year; 2) no, since the deferral wasn't technically due until 4/15/2017 (date company filed it's return), it won't be on the 2016 5500 as a late contribution, but will be on the 2017 5500 as late.  Anyone have a definitive answer, or have an idea of where I can get clarification?

[CuseFan]

Two issues in play: formal written salary reduction election for 2016 for partner should have been executed no later than 12/31/2016 and elective deferral deposit clock started ticking when partner's K-1 was issued, so 4/15 or earlier.

http://ebrworld.com/deferral-election-and-deposit-rules-for-the-self-employed/

I don't do 5500's any more (thankfully!) so I don't know if you would report as late on the 2016 or 2017 return. I would call this a "contingent event" which I'm not sure you have to report, like if the employer adopted a resolution to terminate in 2017 before the 2016 return was filed - I don't think that gets reported. However, if the plan has an audit, I believe the auditors must report contingent events, or at least those that are material, maybe a audit aficionado can chime in here.