A multiprocessor computer system comprises multiple data processors, each with an internal clock for providing time stamps to application software. The processors take turns as synchronization masters. The present master transmits a “request” time stamp (indicating the time of transmission according to the local clock) to the other (“slave”) processors. Each slave processor responds by returning a “response” time stamp (indicating the time of transmission of the response according to the local slave clock) of its own along with the received request time stamp. The master calculates clock adjustment values from the time of receipt of the responses and the included time stamps. This allows asynchronous clocks to be synchronized so that application time stamps can be validly compared across processors.