26 CFR § 1.860C-1 - Taxation of holders of residual interests.
(a) Pass-thru of income or loss. Any holder of a residual interest in a REMIC must take into account the holder's daily portion of the taxable income or net loss of the REMIC for each day during the taxable year on which the holder owned the residual interest.
(b) Adjustments to basis of residual interests -
(3) Adjustments made before disposition. If any person disposes of a residual interest, the adjustments to basis prescribed in paragraph (b) (1) and (2) of this section are deemed to occur immediately before the disposition.
(c) Counting conventions. For purposes of determining the daily portion of REMIC taxable income or net loss under section 860C(a)(2), any reasonable convention may be used. An example of a reasonable convention is “30 days per month/90 days per quarter/360 days per year.”