Deadline to set up new multiemployer plan

Has anyone thought about what the deadline is for setting up a new multiemployer defined benefit plan?

Example:  Plan is intended to be effective July 1, 2017, and to be a calendar year plan.  The employers all have fiscal years ending June 30.  The plan is not finalized until January 20, 2018.

The IRS 401(k) resource guide says, "The plan may not be made effective earlier than the first day of the employer’s tax year in which the plan was adopted. In other words, an employer may adopt the plan document on the last day of its tax year, with an effective date retroactive to the first day of that tax year, but not any earlier."  https://www.irs.gov/retirement-plans/plan-sponsor/401k-resource-guide-plan-sponsors-starting-up-your-plan  If this applies to multiemployer defined benefit plans, it would mean that the plan could be retroactive to July 1, 2017, even if the plan itself has a calendar year.

It would seem odd to make the deadline for adoption of a multiemployer plan the employer's fiscal year.  What if the employers had different fiscal years?  A more reasonable approach would seem to be to have the deadline relate to the plan year--meaning that the plan could be adopted in January of 2018 retroactive to July 1, 2017 only if the plan adopted a plan year that ended between January 31 and June 30.  However, I'm just not finding any guidance at all on this issue.  Is anyone aware of any?

In a multiemployer plan, there is no "employer" per se. There is the plan sponsor, which is a union, and there are contributing employers, which could have differing fiscal years which I believe are irrelevant to the required adoption due date to establish a new plan. What is the sponsoring union's fiscal year? That is the relevant date I believe. if calendar, then i think you have a 2018 calendar year plan and can't go back to 2017. 

27 minutes ago, CuseFan said:

In a multiemployer plan, there is no "employer" per se. There is the plan sponsor, which is a union, and there are contributing employers, which could have differing fiscal years which I believe are irrelevant to the required adoption due date to establish a new plan. What is the sponsoring union's fiscal year? That is the relevant date I believe. if calendar, then i think you have a 2018 calendar year plan and can't go back to 2017. 

A couple of issues with that: 

1. The guidance doesn't say the plan administrator's fiscal year, it says the employer's fiscal year.  (Even a single employer plan can name someone other than the employer as the plan administrator.)  Presumably, this is to prevent employers from fooling around with deduction rules.  But the IRS has not set forth an alternative rule for a plan which has more than one employer (either multiple employer or multiemployer).
2. The plan administrator of a multiemployer  plan is not the union.  It's the joint (union-management) board of trustees.  Which undoubtedly doesn't yet have a fiscal year.  I suppose you could set up the board of trustees to have a different fiscal year than the plan, although that seems like it could cause a lot of confusion.