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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def clean_start_time(self):
 """ Make sure that the start time doesn't come before the active entry """

        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