Abstract: A method and apparatus for updating a regional reference time of a mobile communication terminal by a regional standard time difference according to a Network Identity and Time Zone (NITZ) information change in an asynchronous transfer mode-based mobile communication system are provided. The method of updating a regional reference time of a mobile communication terminal includes determining whether new regional standard time information is received and calculating, if a regional standard time information is changed, a regional standard time difference, which may have a positive or negative value; and resetting the regional reference time by adding the regional standard time difference to a preset regional reference time.