(1) Section 19 of the Principal Act is hereby amended in the definition of "accounting period" in subparagraph (i) of paragraph (aa) (inserted by the Finance Act, 1989) of subsection (3) by the substitution of the following definition for the definition of "accounting period":
"'accounting period' means a period, as determined by the Collector-General from time to time in any particular case, consisting of a number of consecutive taxable periods not exceeding six or such other period not exceeding a continuous period of twelve months as may be specified by the Collector-General:
Provided that -
(I) where an accounting period begins before the end of a taxable period, the period of time from the beginning of the accounting period to the end of the taxable period during which the accounting period begins shall, for the purposes of this paragraph, be treated as if such period of time were a taxable period, and
(II) where an accounting period ends after the beginning of a taxable peri