26 C.F.R. § 1.662(c)-1
If a beneficiary has a different taxable year (as defined in section 441 or 442) from the taxable year of an estate or trust, the amount he is required to include in gross income in accordance with section 662 (a) and (b) is based upon the distributable net income of the estate or trust and the amounts properly paid, credited, or required to be distributed to the beneficiary for any taxable year or years of the estate or trust ending with or within his taxable year. This rule applies as to so-called short taxable years as well as taxable years of normal duration. Income of an estate or trust for its taxable year or years is determined in accordance with its method of accounting and without regard to that of the beneficiary.
26 C.F.R. §1.662(c)-1