To offer round the clock support, it’s now a common practice employ support teams from multiple geographies. Naturally, members of a customer support team might work from various time zones. Tracking tickets in their local time is key to getting things resolved at the earliest.
An agent has to visit the Localization section of the My Settings page to make the change to the default time zone.
Do note that the administrator can set a universal time zone for everyone from the General Settings page (under the Manage section) of the help desk. But, agents can override this default time zone when they opt for a change. Comes in handy when you have a distributed support team!