Calculate due date

Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/billing/management/commands/billing_check.py

@@ -18,8 +18,11 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #
 
+from datetime import timedelta
+
 from django.core.management.base import BaseCommand
 from django.utils import timezone
+
 from weblate.billing.models import Billing
 
 
@@ -40,8 +43,9 @@ class Command(BaseCommand):
                     ' * {0}'.format(bill)
                 )
         header = False
+        due_date = timezone.now() - timedelta(days=30)
         for bill in Billing.objects.filter(state=Billing.STATE_ACTIVE):
-            if not bill.invoice_set.filter(end__gt=timezone.now()).exists():
+            if not bill.invoice_set.filter(end__gt=due_date).exists():
                 if not header:
                     self.stdout.write('Following billings are past due date:')
                     header = True