use float type

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@15698 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/DateUtils.py

@@ -53,7 +53,7 @@ hour = 1/24.
 same_movement_interval = hour
   
 accountable_days_in_month = 30.
-accountable_months_in_year = 12
+accountable_months_in_year = 12.
 number_of_hours_in_year  = 8760
 
 def addToDate(date, to_add=None, **kw):