Steps to reproduce:
have some log file on monitoring with zabbix agent active with key

log[/tmp/log.txt,,,101,skip,,120.0,mtime-noreread]

update the file regurarly

• change time in the system back or forward (it could happen when system is rebooted)

Result:

• agent re-read the file from begin every time despite on options and detecting the time changing and adjusting the mtime:

  26565:20200607:082950.670 the system time has been pushed back, adjusting active check schedule
   26565:20200607:082951.670 System clock has been set back in time. Setting agent mtime 46 seconds back.
   26565:20200607:072950.343 the system time has been pushed back, adjusting active check schedule
   26565:20200607:072951.344 System clock has been set back in time. Setting agent mtime 3675 seconds back.
  

Expected:
Do not re-read the file.