The basic workweek for annual salaried employees of State departments, agencies and institutions, other than those employed on a part-time basis only and other than employees excluded from the application of overtime compensation under the rules of the Director of the Budget adopted pursuant to section 134 of the Civil Service Law, shall be 40 hours; provided, however, that where a shorter workweek would not interfere with the proper performance of government functions an appointing authority may establish a basic workweek of not less than 37 1/2 hours and five days or an authorized equivalent work schedule, for any employees under its appointive jurisdiction as may be designated by it with the approval of the Division of the Budget; provided, however, that this Part shall not be construed to increase the workweek of any employee for whom a workweek of less than 40 hours is prescribed by law, rule or administrative regulation as of the day preceding the effective date of this Chapter.