Adopted Regulation 4203, respecting the calculation of rent decreases, specifying that a rent decrease due to the reduction of housing services or maintenance shall be calculated from the date on which the petition as to which it is granted was filed.

See the Notice on page 2: http://backissues.smdp.com/041719.pdf

Regulations: https://www.smgov.net/uploadedFiles/Departments/Rent_Control/Rent_Control_Law/04,%20Rent%20Adj.pdf