diff --git a/mixin.py b/mixin.py
index e8762fe..8a75aca 100644
--- a/mixin.py
+++ b/mixin.py
@@ -5,7 +5,6 @@
 
 
 from decimal import Decimal
-import datetime
 import html
 from trytond.exceptions import UserError
 from trytond.i18n import gettext
@@ -101,20 +100,19 @@ class EdocumentMixin(object):
         """ get tax of invoice-line,
             fire exception if no/multiple taxes exists
         """
+        Tax = Pool().get('account.tax')
+
         if len(line.taxes) != 1:
             raise UserError(gettext(
                 'edocument_xrechnung.msg_linetax_invalid_number',
                 linename=line.rec_name,
                 numtax=len(line.taxes)))
 
-        tax = line.taxes[0]
-        date = line.invoice.accounting_date or line.invoice.invoice_date
-        for child in tax.childs:
-            start_date = child.start_date or datetime.date.min
-            end_date = child.end_date or datetime.date.max
-            if start_date <= date <= end_date:
-                tax = child
-                break
+        taxlines = Tax.compute(
+            line.taxes, Decimal('1'), 1.0,
+            line.invoice.accounting_date or line.invoice.invoice_date)
+        assert len(taxlines) == 1
+        tax = taxlines[0]['tax']
 
         allowed_cat = ['AE', 'L', 'M', 'E', 'S', 'Z', 'G', 'O', 'K', 'B']
         unece_category_code = self.get_category_code(tax)