48 C.F.R. § 9904.406-50
(a) The cost of an indirect function which exists for only a part of a cost accounting period may be allocated on the basis of data for that part of the cost accounting period if the cost is:
(e) The contracting parties may agree to use an annual period which does not coincide precisely with the cost accounting period for developing the data used in establishing an allocation base: Provided,
(f) When a transitional cost accounting period is required under the provisions of 9904.406-40(a)(3), the contractor may select any one of the following:
(3) A period in excess of a year, but not longer than 15 months, obtained by combining the period described in paragraph (f)(1) of this subsection with the next regular cost accounting period.
A change in the contractor's cost accounting period is a change in accounting practices for which an adjustment in the contract price may be required in accordance with paragraph (a)(4) (ii) or (iii) of the contract clause set out at 9903.201-4(a).