Estimated tax penalty on page 8. Overpaid estimated tax. File Form 4466 before the 16th day of the 3rd month after the end of the tax year, but before Form 1120-FSC is filed. Do not file Form 4466 before the end of the FSC's tax year. Interest and Penalties Interest.
Interest is charged on taxes paid late even if an extension of time to file is granted.
The interest charge is figured at a rate determined under section 6621. Penalty for late filing of return. The penalty will not be imposed if the FSC can show that the failure to file on time was due to reasonable cause. FSCs that file late must attach a statement explaining the reasonable cause. Penalty for late payment of tax.