Method and apparatus to implement a ''virtual'' real-time clock at a terminal based on time information from multiple communication systems. At least one system (e.g., GPS) provides ''absolute'' time information for the virtual real-time clock, and at least one other system (e.g., a cellular system) provides ''relative'' time information. The virtual real-time clock is ''time-stamped'' with absolute time as it becomes available from the first system. Relative time (which may be received from multiple asynchronous transmitters) is mapped to the timeline of the virtual real-time clock as it is received from the second system. Absolute time at any arbitrary time instant on the timeline may then be estimated based on the absolute time from the first system and the relative time from the second system. Absolute times from the first system for two or more time instants may also be used to calibrate the relative time from the second system.