Hi Benny,
Could you try adding a validation rule at the EndDate field with the following formula for the condition:
click condition and select "The expression" from the first dropdown control, be sure to replace the xpath for StartDate with the correct one.
. > xdDate:AddDays(../my:StartDate, "365") and . != ""
The condition woud look similar to this after adding:
EndDate > addDays(StartDate, "365") and
EndDate is not blank
This way, if the EndDate is greater that StartDate + 365 days, a validation error will appear.
I hope this was clear enough. Please let us know if this doesn't work for you.
Regards,