A reminder on the implementation of the calculation formulas for vaildity due dates.
If you sell contributions or federal licenses under OF 3.5, you should replace in "Sales > Validities for sale" hard-coded dates like" 2015-12-31" by the following calculation formula :
formatDate( 'yyyy-12-31', (formatDate('MM', %NOW_DATE)<10) ? %NOW_DATE : changeTime(%NOW_DATE, '+1', 0, 0) )
With this formula, the due date is the end of the year or that of the end of the following year if it is after 1 October.