Filing an extension doesn't change when income tax is due - but it changes the deadline for various things. Not just the 1040 filing deadline. Other procedures

Filing an extension doesn't change when income tax is due - but it changes the deadline for various things. Not just the 1040 filing deadline. Other procedures do not state their own deadline but rely of [the day your tax return is due, including extensions]. And there is no cost or penalty for filing the extension. So you file the extension and sort out the rest later. I have in the past filed the extension (to lock in the extension date), then just days later filed my 1040 - because for once it happened to be ready early. If you file your 1040 package without having filed for extension, you forfeit that later deadline for the rest.