How to move a date by a dynamic duration?

First I want to thank you for your great job on this module. I use it everyday and it rocks!

Using the action "calculate data value", if I set the first value with a field of date type, I can move this date by a fixed duration expressed in seconds, minutes, hours or days. When I use the data selector for this second value, no value is really proposed. I mean I can browse the different available entities, but I cannot select something relevant. What is the type of the second dynamic value supposed to be?

I would like to move the first value by a integer field expressed in days. How can I make that?

Thanks a lot for your help.