(a) The fiscal year of the county shall begin on the first day of January and end on the thirty-first day of December of each year. On or before the fifteenth day of November of each year the controller of each county of the second class shall transmit to the commissioners a proposed budget giving a detailed estimate of and for the legitimate purposes of the county for the next year, including interest due and to fall due on all lawful debts of the county bearing interest. Such budget, when finally adopted by the commissioners, shall be the guide to the commissioners in fixing the tax rate. Said budget shall be prepared as provided herein.

(b) The commissioners shall, at the same time the budget is adopted, fix such rate of taxation upon the valuation of the property taxable for county purposes as will raise sufficient sum to meet the said expenditures. The commissioners shall not, by contract or otherwise, increase the expenditures of the county in any year to an amount beyond the taxes assessed as aforesaid for said year.