The terms and conditions of the appointment letter, by itself, in many cases, normally do not remain in force all the time. There are "Standing Orders" and other Acts to regulate the working hours by category. There are bilateral or tripartite agreements with unions, etc., by which an establishment can change the working hours by mutual agreement. Nowadays, many establishments follow a 5-day workweek, in which case no acts are infringed, and working hours are suitably modified in terms and conditions. Therefore, as long as employees agree, timings are implemented.
Regards,
Kumar S.