The date on which changes in the salaries and rates of pay for county officers and their deputies, aides and assistants under this act¹ due to changes in population, gross assessed valuation or both population and gross assessed valuation in any county shall take effect, shall be as of, on or after the first day of July based upon the population as shown by the latest Federal Decennial Census for the State of Oklahoma, and the total gross assessed valuations of tangible taxable properties for such fiscal year as shown by the official certificate which the county assessor files with the county excise board for the purpose of computing appropriations and levies for such current fiscal year.