California Code of Civil Procedure 2034.230
(a) A demand for an exchange of information concerning expert trial witnesses shall be in writing and shall identify, below the title of the case, the party making the demand. The demand shall state that it is being made under this chapter.
(b) The demand shall specify the date for the exchange of lists of expert trial witnesses, expert witness declarations, and any demanded production of writings. The specified date of exchange shall be 50 days before the initial trial date, or 20 days after service of the demand, whichever is closer to the trial date, unless the court, on motion and a showing of good cause, orders an earlier or later date of exchange.
Source: CCP 2034.230