How to set up Time zone set for staff

How to set up Time zone set for staff as my staff live in different locations. Eg: I add 1 appointment service for clients to book. My time zone is GMT+7 and its time zone for the system (business information), but my staff is living in Madrid, 5 hours behind.
Staff got granted permission, but calendar default is GMT+7, adding working hours is also based on GMT+7.
While clients can adjust booking based on their time zone but my staff is stuck at GMT+7. how can I set Madrid time for him?