(a) A unitrust plan shall provide the period used under Sections 16335 and 16336. Except as otherwise provided in paragraph (3) of subdivision (b) of Section 16338, the period may be any of the following:

1. (1) A calendar year.
2. (2) A 12-month period other than a calendar year.
3. (3) A calendar quarter.
4. (4) A three-month period other than a calendar quarter.
5. (5) Another period.

(b) Except as otherwise provided in Section 16338, a unitrust plan may provide standards for any of the following:

1. (1) Using fewer preceding periods under subparagraph (B) of paragraph (2) of subdivision (a) of, or paragraph (3) or (4) of subdivision (b) of, Section 16335 if any of the following circumstances exists:

   1. (A) The trust was not in existence in a preceding period.
   2. (B) Market indices or other published data are not available for a preceding period.

2. (2) Using fewer preceding periods under subparagraph (A) or (B) of paragraph (5) of, subparagraph (B) of paragraph (6) of, or subparagraph (B) of paragraph (7) of subdivision (b) of, Section 16366, under either of the following circumstances:

   1. (A) The trust was not in existence in a preceding period.
   2. (B) Fair market values are not available for a preceding period.
3. (3) Prorating the unitrust amount on a daily basis for a part of a period in which the trust or the administration of the trust as a unitrust or the interest of any beneficiary commences or terminates.