Steps to reproduce:
Create an item for log file monitoring:

Type: Zabbix agnet (active)
Key: logrt["/path/to/log/file.log",error]
Type: Log
Log time format: MM/dd/yyyy hh:mm:ss

Mikrotik log returns line like:

Nov/02/2017 15:42:39 system,error,critical login failure for user root from [IP] via ssh

If you want to have local log time processed
Create preprocessing rule:

Regular expression: (^Nov)(.+) and as a result: 11\2

Result:
You will get the correct line:

11/02/2017 15:42:39 system,error,critical login failure for user root from [IP] via ssh

But Local time for the log monitoring won't be filed.

Expected:
Local time is processed after item preprocessing which first substitutes human readable months with machine readable MM.