The 1999 Suburban had four different head unit styles available, and all four of them had clock displays. These head units had two different time-setting procedures, depending on whether the head unit had "HR" and "MN" buttons, or not. Setting the time on both versions is a fairly simple task.
Without "HR" and "MN" Buttons
Press and release the "Set" button, and within five seconds hold the right "Seek" arrow until the correct minute shows on the radio. Press and hold the left "Seek" arrow until the correct hour shows on the radio display.
With "HR" and "MN" Buttons
Hold the "HR" button until the correct hour shows on the radio display, then hold the "MN" button until the desired minute shows on the radio display.