Count last 60 seconds of occurrences on a log that rotates hourly

[Kilimbi]

I've set up an system.run item that does a "grep -c 'string' kernel.log".
The kernel.log file rotates hourly and I'd like to have it triggering an alert when the count for 'string' is less than 100 in the last minute.

Option 1: Set up a trigger which counts only the value difference in the last minute.

Option 2: Set up a calculated item which counts only the last minute of the log's entries.

How can I do this? Any delta function maybe?
Basically my intention is to have an item which counts the value as if the log rotation was every minute.

[Kilimbi]

What I wanted to do was solved by simply changing the Store value
= 'As is' to 'Delta' in the item configuration page. The polling was being executed once each 60 seconds so showing the difference from the last and actual value does that.