Usually on older AC-Delco/Remy Radio, press the "set" button and it will take you back to the radio station frequency, then push the left knob inward, then it will take you back to the time again. Before the "set" mode ceases, use the seek and scan buttons to change the minute and hour for the time, until you get to the time you want.
Other models will have the "set" button being a small almost hidden button, you can use a paper clip pin end to depress. Which will initiate the "set" mode, then you can push in the left or right knobs and the time mode will come on. Then using seek rocker switch or the seek and scan buttons the time minutes and hours will start to change.
The last type is where you have to either hold down the set button then use the seek and scan buttons or you have to hold down the set button for about 4 seconds then the clock will go into a blinking or set display mode, for which you can use either the knobs or the seek and scan buttons to produce minute and hour change display on your clock.
Try these methods. I know on older GM vehicles with AC Delco/Remy type radios/stereos, usually there is a "set" button for setting your favorite radio stations to instant numbered buttons, usually up to four or five of them. This same "set" button can be used to set the time. But when you hit the "set" button it will take you to the radio station frequency, not the time. That is when you press in the left knob or sometimes the right knob, and the time display will come on, while it is still in the set mode. Then you can press on the seek and scan buttons, one of them will change your minutes the other one will change your hours, for your clock display time. When you reach the correct time you desire, then hit the "set" button again, which will turn off the set mode. Your clock should start working normally, starting from the time you've set.
Try this, it should resolve your problem.