Invalid result in getting span between two dates

This code: | makeresults | eval StartTime = strptime("2018-01-01 00:00:00", "%Y-%m-%d %H:%M:%S") | eval EndTime = strptime("2018-01-01 00:10:00", "%Y-%m-%d %H:%M:%S") | eval Elapsed = EndTime - StartTime | fieldformat Elapsed = strftime(Elapsed, "%H:%M:%S") results to Elapsed=08:10:00 or eight hours and ten minutes, instead of just ten minutes. Would like to get some assistance on why this happens and how to fix. Thanks a lot!