5500 Due date, with 5558 extension

Brain cramp, and I just want to make sure I'm not crazy. Due to a plan merger, short plan year ends October 10th of 2024. If I'm reading the instructions correctly, for a short plan year, the filing due date is the last day of the 7th CALENDAR month after the end of the short year. This would mean due date is May 31, 2025. Right? Then a 5558 extension would extend the due date to August 15, 2025.

Seems simple, but I'm getting some pushback, and I'm always willing to entertain the possibility that I'm a few cards short of a full deck. 

You read and applied the instructions correctly.  Out of curiosity, what is the pushback you are getting (too soon, too late, day of the month...)?

Thanks Paul. Day of the month, so the "person" is contending due date was May 10th, not May 31...

@Belgarath thanks for the feedback.

I have seen this issue come up several times with respect to short plan years, particularly when the short plan year is a result of a plan termination.  The problem usually surfaces when the plan files a 5558 to extend the date and enters on the form a date that is 9 1/2 months from the end of the short plan year.  That pretty much locks in the due date that is earlier than the latest permissible due date.

Agreed, due date is absolutely last day of the 7th month following PYE (later if extended), but as @Paul I pointed out, if you put an earlier date on the 5558, that is your due date unless you file an updated 5558.  Say what you will about IRS rules, but they do not create 365 potential due dates.