If the credit allowed in any taxable year exceeds a taxpayer's State income tax for that taxable year, the taxpayer may apply the excess as a credit against the State income tax for succeeding taxable years until the earlier of: The full amount of the excess is used; or The expiration of the 15th taxable year after the taxable year in which the Maryland qualified research and development expense was incurred. A tax credit may not be carried back to a preceding taxable year.