September 15 does double duty. It is the extended due date for calendar-year partnerships and S corporations that filed Form 7004 in March, and it is the due date for the third estimated tax payment of the year.
Extended entity returns
Form 1065 and Form 1120-S extended in March are due now. There is no further extension. The per-owner late penalty starts the next day, and because it is computed from the original March date, a return filed in October is already seven months late in the IRS's arithmetic.
Third quarter estimate
The Q3 estimated payment covers June 1 through August 31. It is due September 15 for individuals and for corporations. If your income is uneven, the annualized method on Form 2210 lets you pay less in a light quarter, but you have to file the schedule to claim it.
For owners
Late K-1s from an extended entity land now. If you extended your own return to October 15, you have a month. If you did not, your return is already late.
Common mistake
Paying the Q3 estimate on time and filing the entity return late, or the reverse. Two obligations, two consequences, same day.
Sources
- IRS, About Form 7004
- IRS, Estimated taxes