InfoPath Dev

I'm trying to insert in a form a fields where  is set the previous month, but i've problem to do this, have you a suggestion?

To get the number of the previous month, this formula should do the trick:

(substring(xdDate:Today(), 6, 2) - 1) mod 12 + (substring(xdDate:Today(), 6, 2) = 1) * 12

Can you explain me the mod 12 entry?

I don't understand because you use the (substring(xdDate:Today(), 6, 2) = 1) * 12, maybe i don't undersatnd the logic of mod 12.......

Please forgive my poor know how of xpath.

 I can try to explain how it works if you really need to know, but that formula should work just the way it is without any modification.

 But on second thought, the mod 12 part is unnecessary.  This should work just as well:

(substring(xdDate:Today(), 6, 2) - 1) + (substring(xdDate:Today(), 6, 2) = 1) * 12

this should worhk after the change of the year?

If i change the date of my computer, and now is January that is 1 the previous should be 12 so December it works so. 

 Yes, it should work that way.  Feel free to try it out.

great it works very well, thanks

If do not disturbe can give me a site or a book for to learn the xpath language for to understand well thath you made for me.

also thanks