Deadlines
When federal and state returns are due, what an extension does and does not buy you, and what it costs to miss a deadline outright.
What it costs to miss a deadline outright
The penalty is the only part of the bill with a ceiling. Interest, a forfeited refund and an assessment clock that never starts have no ceiling at all.
October 15: the last day for an extended individual return
Missing the October extension does not make a return one day late. The failure to file penalty is then computed from April 15, and charged per month.
Estimated tax due dates and the safe harbor rule
Estimated tax is due April 15, June 15, September 15 and January 15. Each period is penalized on its own, so a late catch up payment cannot cure an early shortfall.
What an extension buys you, and what it does not
Filing late costs ten times what paying late costs. An extension removes the larger penalty entirely and does nothing at all about the smaller one.
The September 15 deadline for extended business returns
The penalty runs per owner per month, up to twelve months, and it applies when the entity owes no tax at all. One missed filing delays every owner.