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