Section 21A. A town by by-law and a city by ordinance, unless repugnant to the charter of such city, may establish the hours, days and weeks of work and the hours, days and weeks of leave without loss of pay, including, without limiting the generality of the foregoing, holiday leave, vacation leave and sick leave, for any or all employees of such town or city other than those appointed by the school committee; provided, that the number of working hours, days or weeks so established shall not exceed, and the number of hours, days or weeks of leave without loss of pay shall not be less than, the number prescribed by any general or special law applicable to such town or city on the first day of January, nineteen hundred and fifty-two.