Grandstream has hard coded the display of the clock. It has no user configurable options.
It actually drives me nuts.
The clock cannot be positioned, colored, or hidden.
The 12 hour clock always pads the hour to two digits: 01:08 PM.
The doesn't match the rest of the UI.
Grandstream just does not care about these little details. It's why I never recommend them to my high-end clients.