18- 125 C.M.R. ch. 810, § .04
If the taxable years of the members of the unitary business differ, the filing member's (see section .05(A) below) taxable year must be used to determine the net income of the unitary business.
If the precise amount of a unitary member's income can be readily determined from the books for the months involved in the filing member's taxable year, those actual amounts are to be used. In the absence of a precise determination, the income of a unitary member must be converted to conform to the taxable year of the filing member on the basis of the number of months falling within the applicable taxable year. For example, if the filing member operates on a calendar year and a unitary member includible in a combined report operates on a fiscal year ending on April 30, it is necessary to assign 8/12 of that member's income from the current taxable year and 4/12 of the income from the preceding taxable year in order to arrive at a full twelve months' income to be included in the combined report. This method may be used only if the return can be timely filed after the filing member's taxable year ends. As an alternative, the combined report may include the taxable income of a group member for the taxable year ending within the taxable year of the filing member. Once one of these methods is used for a group member, that member must continue to use that method for succeeding years for as long as the corporation remains a member of the unitary business.
After the combined taxable income of the unitary business is determined on the basis of the filing member's taxable year, the apportionment factor must be computed on the basis of the same taxable year.
18- 125 C.M.R. ch. 810, § .04