An appeal to the board shall be deemed filed on the date it is received in the principal office of the board; provided, however, an appeal mailed to the board by means of registered mail shall be deemed filed with the board on the date of the registry with the United State Post Office.
Source:
Section 23081.5, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=BPC§ionNum=23081.5..