or taxable year, and (3) with respect to the application of the credit under subsection (a) to third-party payors (including professional employer organizations, certified professional employer organizations, or agents under section 3504 of the Internal Revenue Code of 1986), including regulations or guidance allowing such payors to submit documentation necessary to substantiate the eligible employer status of employers that use such payors. (k) Application This section shall only apply to calendar quarters beginning after December 31, 2019, and before January 1, 2022.