Yes, it is legal. An employer is not required by law to reimburse employees for mileage in the first instance--it is completely voluntary whether they do this. Therefore, since it is voluntary, employers may put conditions on reimbursement--such as requiring that the paperwork be turned in prior to a certain date. Thus, as a general matter, it would be legal for an employer to decline to pay reimbursement which is submitted too late.The exception would be if there had been an agreement in place between you the employer, either governing reimbursement generally or governing reimbursement for the particular mileage in question (e.g. they had agreed to reimburse you a particular trip, with no restrictions). In that event, the employer must honor the terms of the agreement.