The due date to file Form 3520 is the 15th day of the 4th month following the end of the U.S. owner's tax year. The instructions to Form 3520 explain that if the U.S. owner is granted an extension of time to file Form 1040, the due date for filing Form 3520 is the 15th day of the 10th month following the end of the U.S. person's tax year. In other words, Form 4868 extends the time to file Form 3520 along with Form 1040.

The instructions to Form 3520-A state that Form 3520-A must be filed by the 15th day of the 3rd month after the end of the trust's tax year. A six-month extension of time to file Form 3520-A may be granted by filing Form 7004, "Application for Automatic Extension of Time To File Certain Business Income Tax, Information, and Other Returns."

If you are filing "substitute" Form 3520-A and attaching it to Form 3520, the due date for substitute Form 3520-A is the same as Form 3520. See a blog article on this at www.taxconnections.com/taxblog/foreign-trust-irs-penalty-notices-for-late-forms-3520-a-are-scaring-innocent-taxpayers/?.

The due date to file a Form 3520 and a "substitute" 3520-A is the same as your tax return which is April 15th. If a timely extension is filed then the due date of these forms is October 15th. A 3520-A which is not a substitute is filed within 3 months and 15 days of the end of the trust year. An extension for 6 months can be filed for this as well.