5 C.F.R. § 11.4 – Crediting service.

(a) Prior Federal civilian service (including nonappropriated fund service) counts toward completion of a probationary or trial period, as applicable, when the prior service:
1. (1) Is in the same agency, e.g., Department of the Army;
2. (2) Is in the same line of work, as determined by the employee's actual duties and responsibilities; and
3. (3) Contains or is followed by no more than a single break in service that does not exceed 30 calendar days.
(b) Periods of absence while in a pay status count toward completion of a probationary or trial period. Absence in nonpay status while on the rolls (other than for compensable injury or military duty) is creditable up to a total of 22 workdays. Absence (whether on or off the rolls) due to compensable injury or military duty is creditable in full upon restoration to Federal service. Nonpay time in excess of 22 workdays extends the probationary period or trial period by an equal amount.
(c) The probationary or trial period for part-time employees is computed on the basis of calendar time, in the same manner as for full-time employees. For intermittent employees, i.e., those who do not have regularly scheduled tours of duty, each day or part of a day in pay status counts as one day of credit toward the 260 days or 520 days, as applicable, in a pay status required for completion of a probationary or trial period. Under no circumstances shall the probationary or trial period be completed in less than 1 year of calendar time.