code: def clean_start_time(self):
          
          start = self.cleaned_data.get('start_time')
          if not start:
              return start
          active_entries = self.user.timepiece_entries.filter(
              start_time__gte=start, end_time__isnull=True)
          for entry in active_entries:
              output = ('The start time is on or before the current entry: '
                        '%s - %s starting at %s' % (entry.project, entry.activity,
                                                    entry.start_time.strftime('%H:%M:%S')))
              raise forms.ValidationError(output)
          return start
docs: Make sure that the start time doesn't come before the active entry