California Code, Unemployment Insurance Code - UIC § 13042

A new withholding exemption certificate filed under this division in cases in which a previous certificate was in effect shall take effect with respect to the first payment of wages made on or after the first status determination date which occurs at least 30 days from the date on which such certificate is so furnished, except that at the election of the employer such certificate may be made effective with respect to any payment of wages made on or after the date on which such certificate is so furnished.

For purposes of this section, “status determination date,” means January 1, May 1, July 1, and October 1, of each year.