The deadline for extended calendar-year business, investment and pass-through tax returns is Friday, Sept. 15.

According to New Canaan financial expert Roy Abramowitz, it's important to pay on time due to interest and penalties on "underpayment of estimated taxes" being around 7%.

"Even if you don't have all the information to do a correct return, file your return because there are severe penalties for late filing, even on a pass-through entity. When you gather the information, you can file an amended return and there will not be any penalties because you filed on time," says Abramowitz.

He says third quarter estimated taxes are also due on Friday.