diff --git a/shop/management/commands/invoice-worker.py b/shop/management/commands/invoice-worker.py
index 8bf95170..f1e7c518 100644
--- a/shop/management/commands/invoice-worker.py
+++ b/shop/management/commands/invoice-worker.py
@@ -1,6 +1,7 @@
 from django.core.management.base import BaseCommand
 from django.core.files import File
 from django.conf import settings
+from django.utils import timezone
 from shop.pdf import generate_pdf_letter
 from shop.email import send_invoice_email
 from shop.models import Order, Invoice
@@ -11,17 +12,20 @@ class Command(BaseCommand):
     args = 'none'
     help = 'Send out invoices that have not been sent yet'
 
+    def output(self, message):
+        self.stdout.write('%s: %s' % (timezone.now().strftime("%Y-%m-%d %H:%M:%S"), message))
+
     def handle(self, *args, **options):
-        self.stdout.write('Invoice worker running...')
+        self.output('Invoice worker running...')
         while True:
             # check if we need to generate any invoices
             for order in Order.objects.filter(paid=True, invoice__isnull=True):
                 # generate invoice for this Order
                 Invoice.objects.create(order=order)
-                self.stdout.write('Generated Invoice object for order %s' % order)
+                self.output('Generated Invoice object for order %s' % order)
 
             # check if we need to generate any pdf invoices
-            for invoice in Invoice.objects.filter(pdf__isnull=True):
+            for invoice in Invoice.objects.filter(pdf=''):
                 # put the dict with data for the pdf together
                 formatdict = {
                     'invoice': invoice,
@@ -34,14 +38,14 @@ class Command(BaseCommand):
                         template='invoice.html',
                         formatdict=formatdict,
                     )
-                    self.stdout.write('Generated pdf for invoice %s' % invoice)
+                    self.output('Generated pdf for invoice %s' % invoice)
                 except Exception as E:
-                    self.stdout.write('ERROR: Unable to generate PDF file for invoice #%s. Error: %s' % (invoice.pk, E))
+                    self.output('ERROR: Unable to generate PDF file for invoice #%s. Error: %s' % (invoice.pk, E))
                     continue
 
                 # so, do we have a pdf?
                 if not pdffile:
-                    self.stdout.write('ERROR: Unable to generate PDF file for invoice #%s' % invoice.pk)
+                    self.output('ERROR: Unable to generate PDF file for invoice #%s' % invoice.pk)
                     continue
 
                 # update invoice object with the file
@@ -52,11 +56,11 @@ class Command(BaseCommand):
             for invoice in Invoice.objects.filter(sent_to_customer=False).exclude(pdf=''):
                 # send the email
                 if send_invoice_email(invoice=invoice):
-                    self.stdout.write('OK: Invoice email sent to %s' % invoice.order.user.email)
+                    self.output('OK: Invoice email sent to %s' % invoice.order.user.email)
                     invoice.sent_to_customer=True
                     invoice.save()
                 else:
-                    self.stdout.write('ERROR: Unable to send invoice email for order %s to %s' % (invoice.order.pk, invoice.order.user.email))
+                    self.output('ERROR: Unable to send invoice email for order %s to %s' % (invoice.order.pk, invoice.order.user.email))
 
             # pause for a bit
             sleep(60)