Be more specific, what Navigation unit do you have?
On C-MAX-es with stock navigation, they usually use TravelPilot Ex, from Blaupunkt. If you have it too, then you should have 2 clock displays: one in the instrument panel (above the computer display, the one with the mileage and temperature & rest) and one on the TravelPilot Ex head-unit. The one in the instrument panel can't be change (at least I didn;t succeded).
Regarding the one on the head-unit, pres the Menu key twice and you should see a menu "Clock". Press the corresponding button and you should be able to adjust the time zone (GMT+x) and the Summer/Winter hour. The actual timeis received from the GPS satellite and can't be changed. You can change it only if there is no GPS signal (right after you start the car or when you are in an underdround parking lot) but as soon as the GPS signal is restored, your settings will be overrided.
to get correct time on cmax fitted with sat nav. disconect negative battery terminal,when reconected the clock will show 3pm or 1500hours.So if you reconect at 3pm you will have the correct time.Bit ponderous but its the only way i can find.you will have to key in your audio code after you reconect battery,should be in your paper work.