Dear camu,
Thank you very much for your attention.
The version of SlimStat I am using is 2.8.2
Upon reading your message, I checked the SlimStat changelog.
I found there have been many updates after 2.8.2. thanks to your time and effort.
And I found “Fixed: bug in calculating visit durations” on the version of 2.8.6. So, I think I can solve the problem with plugin upgrade.
I will upgade the SlimStat to newest version.
Thank you very much for your help.
Best Regards,