XyMon msg metric - how to check log entries only since last check?

Is it possible to setup xymon to check log-file (msg metric) only for new entries since last check (or 5 min)?
For now I see it is only possible to specify how much data in log-file xymon will parse.
But, depending on activities it can be big or small - so the same bad entry can be reported for 2 or more check intervals and some (when quite much activities in log-file) can be missed.

Really I do not realise how it can be implemented w/o writing timestamp tags to that log-file,
but may be You know some secret?

-- 
Best regards,

Andrey Chervonets
SIA CoMinder
www:http://www.cominder.eu/