Government employees who are required to work on holidays, except Thursday half-holidays, shall be compensated by extra pay or time off for such work in accordance with existing overtime laws and regulations; Provided, That in the case of employees whose employment on Sundays on a regular or rotating shift of duty basis is necessary in the public interest, including employees in public institutions such as the government hospitals or the institutions for the aged, or employees engaged in work relating to the public safety, health or fire prevention, such employment on Sundays shall not be deemed work on holidays for the purposes of extra pay or time off as provided for in this section.