(a) The calculation of mileage shall be determined, annually, as of the first day of January by the department from reports submitted by the municipalities.

(b) The calculations involving population shall be made by the department, and shall be based on the latest available official census figures determined on or before the first day of January in the year the money is to be paid to the municipalities. Where the population of any municipality changes by reason of annexation, withdrawal or dissolution, the department shall not be presumed to have notice of such change until furnished with a statement approved by all affected political subdivisions or with an order of a court of competent jurisdiction.