A data communications system (100) that communicates messages by way of a radio frequency (RF) channel between a network control processor (NCP 102) and subscriber radios (190) located anywhere in a large geographical area that is divided into a plurality of cells or zones, each of which is covered by one or more base station radios (130-132). Base station radios (130-132) and a control station radio (140) are coupled by corresponding general communications controllers (GCCs 120-123) and modems (108-111) via links (150-152) to modems 104-107) coupled to the NCP (102). The control station radio (140) transmits a set-clock message (610) to GCCs (120-122) associated with each of the base station radios (130-132). Each GCC (120-122) is responsive to receipt of the set-clock command for updating its clock (214). Each data message (600) to be transmitted to the subscriber radios (190) may be substantially simultaneously transmitted or "simulcasted" at a pre-selected time. Each data message (600) sent by the NCP (102) to GCCs (120-122) includes an address/command portion (602), a data portion (603) including XTIME, the pre-selected time of transmission, and an optional information portion (604). Each GCC (120-122) compares the XTIME of a data message to the time indicated on its clock (214) to determine when to transmit the data message. As a result, all base stations (130-132) receiving the same data message (600) from the NCP (102) simulcast that data message at as pre-selected time XTIME.