Sec. 6.03. CONTRIBUTIONS BY MUNICIPALITY. (a) Subject to modification under Section 11.01 of this Act and not later than the 15th business day after the first day of the municipality's fiscal year, the municipality shall contribute to the fund 18 percent of payroll based on authorized positions, as determined by the municipality.

(b) Not later than December 31 of the year following the year in which the municipality makes a contribution under Subsection (a) of this section, the municipality shall:

(1) calculate the difference, if any, between the amount of the municipality's actual payroll for the applicable fiscal year and the amount of payroll on which its contribution under Subsection (a) of this section was based; and

(2) contribute to the fund an amount equal to the municipality's applicable contribution rate multiplied by the amount of the difference calculated under Subdivision (1) of this subsection.