Data log timestamps

Hi folks, I noticed that rgw data logs are generated with a timestamp (cls_log_entry in cls_log_types.h), which seems to be used to ensure a certain order among all logs from peers in a zonegroup during sync. Since machine clocks are not perfectly in sync and using timestamps from different machines may lead to the different order than what truly has happen, how does ceph/rgw protect itself against this issue? Originally, I thought it gets a monotonic ID from the master zone or using master zone's timestamp. It doesn't seem to be the case or did I read the code wrong? Thanks,Yixin