Provided are an apparatus and a method for setting time information of an area in which a portable terminal, used in an asynchronous mobile communication system, is positioned. The apparatus includes a communicator for transmitting a time information setup request message to a Base Station (BS) and receiving a time information setup complete information from the BS, and a controller for generating the time information setup request message by changing a message type of a Mobility Management (MM) information message and setting the time information of the portable terminal using the time information setup complete message.