How to have cumulative alarms time in report

How to have cumulative alarms time in report

I need to do a report where weekly or montly display, for some alarm, count and total time (active property).
With count no problem
About time, I extract the singol "active" data from alarm, but I didn't find the way to sum this data. The problem, I think, is because extracted is a string. There is any way to convert the data in order to have this total time?
I hope to be clear enought
Thanx

This is a migrated post! Originally posted on 20.05.2008 by user fabios. Please be aware that information can be outdated.