Off PYE with Fiscal Tax filing - deadline?

Have a client that had their fiscal year end and plan year end 9/30. They changed fiscal year end to 12/31 but didn't change the plans year-end yet. They have a Safe Harbor 3% due for the 9/30/22 plan year end. When does it have to be funded by now that their fiscal year is 12/31?

The 9/30/2022 PYE & FYE is due by tax return due date for that 9/30 FYE, including extensions. Going to a 12/31 FYE and having a short tax year 10/1-12/31/2023 doesn't change that I wouldn't think.

the SH is due within 12 months after the plan year ends, regardless of what year's tax return they deduct it on (although, subject to 404 limitations if they tried to deduct it in a short tax year)

You need to check the compensation determination period in the plan document as it may have different results for 404 deductions. This could be an issue with the 25% deduction especially if you have multiple plan years deducted in one fiscal year.

However everything else is based on the plan year i.e. testing, contribution deadlines etc.