Open Source Document Management System | OpenKM

Hello everyone.
I'm a newbie to OpenKM and I'm trying to creating a calculated filed that returns a difference in days of two date fields in a same property group.

Let's say that I have a field named firstdate and another field named lastadate. Now I need to know how many days are between them.
As far as I know in SQL I can do this: SELECT DATEDIFF(lastdate,firstdate)

I'm not a expert programer and I need some help to do this in OpenKM.

Thank you

Actually this kind of feature can not be solved in a easy way. Is needed some change in source code to get it. Basically I think the most easies way for doing it is extending automatic task -> event change metadata. And each time metadadata is updated then calculate it.