For a number of taxpayers September 15 is a key deadline.
Your third installment of federal estimated taxes for 2011 is due by September 15.
You may be subject to federal estimated taxes if you do not have enough federal withholding deducted from your income. This can happen if the bulk of your income is from self-employment, investment income including capital gains or sundry income such as gambling winnings.
If you do not pay sufficient estimated tax during the year you will be charged an estimated tax penalty on your next federal tax return.
Depending on your state tax rules you may also have state estimated taxes to pay.
Partnership 1065 Returns
If your partnership has a calendar tax year and you filed an extension back in April then you need to file the 2010 1065 Return of Partnership Income by September 15. To complete the return you will need to finalize your accounting records for the 2010 calendar year.
Filing of the 1065 after September 15 will lead to a late filing penalty of $195 per month the return is late times the number of partners. This will be charged for up to 12 months so late filing can get expensive very quickly.
Corporation 1120 or 1120S Returns
If your corporation has a calendar tax year and you filed an extension back in March then you need to file the 2010 1120 Corporation Tax Return or 2010 1120S Income Tax Return for an S Corporation by September 15. To complete either return you will need to finalize your accounting records for the 2010 calender year.
Filing of the 1120S return after September 15 will lead to a late filing penalty of $195 per month the return is late times the number of shareholders. To this is added 5% of any outstanding tax up to a maximum of 25%. The form 1120 is only subject to the 5% tax related penalty.
The rules above for partnerships and corporations also apply to LLCs if they have elected to be taxed as one of those entities or are taxed by default as one.