Determination of Dates - xml help

Dear all,

I am struggling with determining a duration in a date profile.

I have found some explanations on a wiki but I am not sure what I need to do is possible with standard.

I have declared 2 dates, let's say TODAY and BIRTHDAY . I would like to have the duration between those 2 dates, so that i can know how many days I have left until the day of the BIRTHDAY. The duration can be minus which means that the Birthday dates is in the past.

Let's say today is 29/08/2014

Birthday is 30/08/2014

I would like my duration to say : 1

If we are today : 01/09/2014 - I would like my duration to be -2

Would anyone be kind on sharing the possibility of xml code for this kind of rule?

I have found the explanation below but have no idea where to put the 'dates' to make the calculation.

Best regards and thanks again.

Anaïs