The interest rate that individuals will pay on underpayments remains at 3% for the first quarter of 2012. The interest rate that the IRS will pay on overpayments (tax refunds) will also be 3%. Most ordinary tax refunds do not earn interest. The IRS does not have to pay interest on tax refunds made within 45 days of the return
A 12-month period ending on the last day of any month other than December. Partnerships, S corporations, and personal service corporations are limited in their choice of fiscal years and face special restrictions.