Mitsubishi Lancer 1997: When should the timing belt be replaced?

I OWN a 1997 Mitsubishi MR Lancer CE that has been serviced every six months by a Mitsubishi outlet and has done 48,000km. The service book indicates the timing belt should be changed on the basis of kilometres or months, whichever comes first. The next scheduled service is 72-months/90,000km, at which the timing belt is scheduled to be replaced. Mitsubishi's service people say the timing belt should be replaced, but other people say it should only need replacing once the car has reached the kilometres specified. What is your opinion?