Tax Extension DeadlineThe year, in order
The night before search starts with one date and finds a year of them. These are the ordinary calendar year dates. If the fifteenth lands on Saturday, Sunday, or a federal holiday, filing moves to the next open weekday.
Walk your rowsThe federal rows
Middle of March, partnership and S corp returns, or the extension that moves them to the middle of September. Middle of April, individual and C corp returns, or the extensions that move them to the middle of October. Quarterly estimates keep four dates of their own, and an accepted extension leaves those dates alone. The middle of October is the end of the individual extension. There is no ordinary second extension after it.
The IRS extension page is the source for the individual window. The page about Form 7004 is the source for the entity window. We keep those two sources separate on purpose, because a reader who blends them files the wrong form in the right month and feels cheated by the calendar.
The row a federal list leaves off
A state income return, where the state has one, with its own request for more time if you need one. Copying Form 4868 into a state packet is how a quiet state grows a filing it never published. A calendar that adds one state extension for every reader is adding a filing some states do not publish. The state's own instructions are the source for that day and for the penalty shape on a late state return. The state revenue page for this filing year is the one we follow. A figure copied from a September reading can be a year stale by the time you file.
Why the list is only useful with a month under it
A date with no closed month under it is a reminder, not a filing. The last column that matters is the one nobody prints on a poster. What has to be tied before that row arrives. We keep that month, and we write an October plan before the year is over, so the next March is a confirmation instead of a reconstruction.
If one of the rows is already behind you, say which one. The morning-after page is the order that costs less. The list on this page does not scold. It only refuses to pretend a missed row is still in the future.