Maximum and minimum @timestamp value

Is there a way to convert @timestamp from iso8601 to unix seconds in log stash so that i can use timer in metrics function? I want minimum and maximum @timestamp value.
Also it would work if I know the first and last timestamp value as log file is always chronologically arranged.

Metrics timer wants a duration (I believe in seconds), not an absolute time. If you always want a duration relative to the @timestamp for events you are targetting, I would try a ruby filter.