5 C.F.R. § 630.1304 – Eligibility.

(a) An employee who is a veteran with a qualifying service-connected disability is entitled to disabled veteran leave under this subpart, which will be available for use during the 12-month eligibility period beginning on the first day of employment. For each employee, there is a single first day of employment.
(b) In order to be eligible for disabled veteran leave, an employee must provide to the agency documentation from the Veterans Benefits Administration certifying that the employee has a qualifying service-connected disability. The documentation should be provided to the agency—
1. (1) Upon the first day of employment, if the employee has already received such certifying documentation; or
2. (2) For an employee who has not yet received such certifying documentation from the Veterans Benefit Administration, as soon as practicable after the employee receives the certifying documentation.
(c) Notwithstanding paragraph (b) of this section, an employee may submit certifying documentation at a later time, including after a period of absence for medical treatment, as described in § 630.1306(c). The 12-month eligibility period is fixed based on the first day of employment and is not affected by the timing of when certifying documentation is provided.
(d) If an employee's service-connected disability rating is decreased or discontinued during the 12-month eligibility period such that the employee no longer has a qualifying service-connected disability—
1. (1) The employee must notify the agency of the effective date of the change in the disability rating; and
2. (2) The employee is no longer eligible for disabled veteran leave as of the effective date of the rating change.