The timing starts with a gun shot, gun time, and the first start group begins the race. The clock by the finish line displays the time that has passed since the first start group began the race. Five minutes elapse between start groups being started. The team that is in charge of the timing corrects this time difference as soon as participants in other start groups then the first one cross the finish line. The order that participants cross the finish line in therefore need not reflect exactly their order in the results.
See further information about the start groups here.