Assuming its a calendar year and you filed the extension prior to 3/15, then the due date is 9/15.

The original deadline was March 16  (as March 15 was a Sunday).  As this was before, the IRS changed certain deadlines due to Covid-19, that date reminded.  If it is on extension, September 15 is the deadline 

March 15 was a Sunday, so it was March 16.  But I think the person was probably asking because of the deadlines that changed due to Covid 19