An additional charge of two percent of the unpaid tax shall be imposed per month for each month or fraction of the month as interest on any tax not paid on or before the dates specified in section 901 of this charter. (Amended 2003, No. M-13 (Adj. Sess.), § 3; 2009, No. M-18 (Adj. Sess.), § 12, eff. May 5, 2010;)