Answer from Kyle, PHR:

Don’t wait. If the employee was underpaid, it is in your best interest to pay the employee the difference immediately. Wages are due on the regular payday for the pay period covered, as required by the Fair Labor Standards Act. State law may apply as well. When an error occurs and results in an underpayment, the employer is technically in violation of the law, even if it was a system error or caused by an employee’s timekeeping mistake.

This Q&A does not constitute legal advice and does not address state or local law.