Forms must be submitted by midnight to avoid penalty.

Procrastinators, don't despair. There is still time to file your taxes — but not much.

Tuesday is the deadline to file federal and Massachusetts tax returns and, if taxes are owed, to do so without incurring a penalty. Any forms must bear a Tuesday postmark or be submitted electronically before midnight to be considered on time.

John Clisham, a tax preparer with H&R Block in Hyannis, said about one-third of all taxpayers file their returns in April. Extensions are available to push the deadline to Oct. 15, but that does not excuse the need to make a payment if one is required.

"If you file for an extension until Oct. 15, you actually have to pay something today," he said. The IRS requires 90 percent of the tax liability for the year or, if that is unknown, then 100 percent of last year's liability.

According to a statement from the IRS, taxpayers may pay online, by phone, check or money order or use a credit card or obtain a loan to pay taxes, options that may carry lower interest rates than the penalties and interest associated with a late payment. The IRS charges 5 percent per year, compounded daily, on any unpaid balance; the late-payment penalty is 0.5 percent per month. Taxpayers also may set up a payment plan online to pay their bill over time.

Clisham said an amended return could be filed if a taxpayer could not find all the proper paperwork by Tuesday but wanted to get a return sent. The biggest mistake people can make, he said, is deciding that not filing is a better alternative than facing a tax bill.

"You would be surprised at the number of taxpayers that think that," he said. "The IRS knows a lot about us, and does invoke penalties and interest."

Clisham said many taxpayers he has worked with have had questions about the new tax laws passed in 2017 and how they will affect what is owed next year. The law nearly doubles the standard deduction and increases child tax credits while eliminating several exemptions and credits; it also created new tax brackets and lowered the tax rate for most income ranges.

"It's confusing, and a lot of people have questions," Clisham said.

The timing of the law's passage shortly before the new year meant tax preparers were thrust straight into tax season before they could get a thorough grasp on the law's implications, Clisham said. While he has been trying to explain what will be different to people who have questions, he said it was too early to say for sure how tax returns would look in 2019.

"Congress could still change it," he said.

— Follow Sean F. Driscoll on Twitter: @seanfdriscoll.