§ 21. Office hours. Unless otherwise provided by law, the city offices shall be kept open for the transaction of business each day in the year, Sundays and legal holidays excepted, from ten o'clock in the forenoon until four o'clock in the afternoon, except that the offices of comptroller, city treasurer and the office for the collection of water rents, shall be kept open as aforesaid from ten o'clock in the forenoon until three o'clock in the afternoon. The common council may from time to time direct any city office to be kept open at such other hours as public convenience may require.