d43df8b8_ehire__or_Interagency_Transfer__then_
[If…] you are correcting the employment date (ASSIGN DT) [then…] 
You may change the employment date to a date in the current or previous month. You may not change the employment date to a date before the first day of the previous month.
USPS Process Guide: Ch 4, HMPFX - Changing Information for a New Hire, Rehire, or Interagency Transfer
then…