diff --git a/modules/purchase/purchase.py b/modules/purchase/purchase.py
index 5b00578..ba31030 100755
--- a/modules/purchase/purchase.py
+++ b/modules/purchase/purchase.py
@@ -519,6 +519,8 @@ class Purchase(
 
     @property
     def taxable_lines(self):
+        Date = Pool().get('ir.date')
+        tax_date = getattr(self, 'purchase_date', None) or Date.today()
         taxable_lines = []
         # In case we're called from an on_change we have to use some sensible
         # defaults
@@ -529,7 +531,7 @@ class Purchase(
                     getattr(line, 'taxes', None) or [],
                     getattr(line, 'unit_price', None) or Decimal(0),
                     getattr(line, 'quantity', None) or 0,
-                    None,
+                    tax_date,
                     ))
         return taxable_lines
 
diff --git a/modules/sale/sale.py b/modules/sale/sale.py
index 834198d..43fc849 100755
--- a/modules/sale/sale.py
+++ b/modules/sale/sale.py
@@ -1596,11 +1596,14 @@ class SaleLine(TaxableMixin, sequence_ordered(), ModelSQL, ModelView):
         # In case we're called from an on_change
         # we have to use some sensible defaults
         if getattr(self, 'type', None) == 'line':
+            Date = Pool().get('ir.date')
+            sale = getattr(self, 'sale', None)
+            tax_date = getattr(sale, 'sale_date', None) or Date.today()
             return [(
                     getattr(self, 'taxes', None) or [],
                     getattr(self, 'unit_price', None) or Decimal(0),
                     getattr(self, 'quantity', None) or 0,
-                    None,
+                    tax_date,
                     )]
         else:
             return []