California Code, Penal Code - PEN § 905.5

(a) Except as otherwise provided in subdivision (b), the grand jury shall be impaneled and serve during the fiscal year of the county in the manner provided in this chapter.

(b) The board of supervisors of a county may provide that the grand jury shall be impaneled and serve during the calendar year. The board of supervisors shall provide for an appropriate transition from fiscal year term to calendar year term or from calendar year term to fiscal year term for the grand jury. The provisions of subdivisions (a) and (b) of Section 901 shall not be deemed a limitation on any appropriate transition provisions as determined by resolution or ordinance; and, except as otherwise provided in this chapter, no transition grand jury shall serve more than 18 months.