This is assuming the use of a monotonic clock. the system wall clock may jump backwards in time. Itt also assumes that only one id can be generated per millis

This is assuming the use of a monotonic clock. the system wall clock may jump backwards in time. Itt also assumes that only one id can be generated per millisecond (or whatever clock resolution). In the case of multiple threads or processes on multiple cores, it becomes more likely that there will be a collision. Timestamps don't make for a great ordering key.