(a) (1) Interest begins to accrue on the date or dates (usually one or more pay dates) on which the employee would have received the pay, allowances, and differentials if the unjustified or unwarranted personnel action had not occurred. (2) Interest accrual ends at a time selected by the agency that is no more than 30 days before the date of the back pay interest payment. No interest is payable if a complete back pay payment is made within 30 days after any erroneous withdrawal, reduction, or denial