jtreg reports the start and end date time in each action's section in the jtr file. For example:

#section:main
----------messages:(9/323)----------
command: main FooBar
...
started: Fri Aug 22 09:42:31 UTC 2025
Mode: agentvm
Agent id: 20
Process id: 12345
....
finished: Fri Aug 22 09:43:47 UTC 2025
elapsed time (seconds): 75.919

Both the start and end times do not report the millisecond part. If/when a test fails, this detail can some times be useful during analysis. For example, so other log files like the agent server's log file and the harness trace file contain timestamp that include the millisecond part. Having that same level of detail in the start/end date time reported for each action can some times help during debugging when comparing these logs.