The new year may bring changes to your business tax filing dates, we have provided a list of important dates to remember to make sure you are compliant. (rocketlawyer.com)
If your 180 day comes after tax filing date, you must file for extension on your tax return in order to get the full 180 (biggerpockets.com)
If you're 50 on Dec 31st of the tax year, you can make the full catch - up contribution by Tax filing date of the following year (or anytime during the year, doesn't matter) (money.stackexchange.com)