An employee must meet all of the following conditions to be eligible to receive a post differential:
(a) The employee must be a citizen or national of the United States,
(b) The employee's official worksite or detail to temporary duty must be in the post differential area, and
(c) Immediately prior to being assigned to duty in the post differential area, the employee must have maintained his or her actual place(s) of residence outside the post differential area for an appropriate period of time (generally at least 1 year or more), except as provided in § 591.234.