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.
Estimated tax due dates, and the safe harbour that avoids a penalty
The penalty is worked out separately for each period, so paying extra later does not undo an earlier shortfall. Withholding is the exception, and it is useful.
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.