Rather than limiting a person to a timezone, which is a handle feature assuming everyone works business hours, and then goes on-call. For 24/7 onsite times this not the case. For example: if a second shift (16:00 - 00:00) is called, it will show as after hours or sleep time, when in fact those are that individuals working hours. Conversely if this person is contacted at 10:00am that may be that person’s sleep hours, rather than normal business hours.
A extra feature in the user profile could be “Working Hours” - This would go one step further in the reporting and trying reflect on call impact for all responders.