code: def start_of_day(dtime_at=None):
      
      dtime_at = datetime_or_now(dtime_at)
      return datetime.datetime(dtime_at.year, dtime_at.month,
          dtime_at.day, tzinfo=tzlocal())
docs: Returns the local (user timezone) start of day, that's,
      time 00:00:00 for a given datetime