(a) After 24 months of continuous service outside the United States (or after a shorter period of such service if the employees assignment is terminated for the convenience of the Government), an employee may be granted leave of absence, under regulations of the President, at a rate not to exceed 1 week for each 4 months of that service without regard to other leave provided by this subchapter. Leave so granted
(1) is for use in the United States, or if the employees place of residence is outside the area of employment, in its territories or possessions including the Commonwealth of Puerto Rico;
(2) accumulates for future use without regard to the limitation in section
6304 (b) of this title; and
(3) may not be made the basis for terminal leave or for a lump-sum payment.
(b) The President may authorize leave of absence to a chief of mission excepted from this subchapter by section
6301 (2)(xii) of this title for use in the United States and its territories or possessions. Leave so authorized does not constitute a leave system and may not be made the basis for a lump-sum payment.
(c) An officer, crewmember, or other employee serving aboard an oceangoing vessel on an extended voyage may be granted leave of absence, under regulations of the Office of Personnel Management, at a rate not to exceed 2 days for each 30 calendar days of that service without regard to other leave provided by this subchapter. Leave so granted
(1) accumulates for future use without regard to the limitation in section
6304 (b) of this title;
(2) may not be made the basis for a lump-sum payment, except that civil service mariners of the Military Sealift Command on temporary promotion aboard ship may be paid the difference between their temporary and permanent rates of pay for leave accrued under this section and section
6303 and not otherwise used during the temporary promotion upon the expiration or termination of the temporary promotion; and
(3) may not be made the basis for terminal leave except under such special or emergency circumstances as may be prescribed under the regulations of the Office.