US tax return filing deadlines for partnerships and C corporations were changed by Congress in late July under a bill to extend funding for the highway trust fund.

The changes will apply to tax returns starting with the 2016 tax year.

Partnerships that use the calendar year as their tax year will have to file returns by March 15 rather than April 15 as under current law. Partnerships with fiscal years will have to file by the 15th day of the fourth month after the fiscal year ends. Partnerships will be able to get extensions of up to six months.

Calendar-year corporations will have until April 15 to file, an extra month beyond the current deadline of March 15. Fiscal-year corporations will have until the 15th day of the fourth month after the fiscal year ends. However, corporations with fiscal years ending June 30 will not be affected by the changes until the 2025 tax year.

From 2016 through 2026, calendar-year corporations will only be able to extend the due date by five months through September 15. During the same period, corporations with June 30 fiscal years will be able to extend by seven months. All other fiscal-year corporations will be able to extend by six months.

The bill also tinkered with the statute of limitations for the IRS to assess back taxes.

The IRS usually has only three years after a return is filed to assess back taxes. However, it has six years where the taxpayer underreported income by 25% or more. The US Supreme Court held in a case called Home Concrete & Supply in 2012 that underreporting of gain on the sale of property, because the taxpayer claimed too high a tax basis, does not bring the six-year statute of limitations into play. The bill says that such gain should be taken into account. The change applies to tax returns filed after July 31, 2015 and to any previously filed returns that are still open for assessment.