docs: def calculate_start_date(display_since):
      """
      Private function
      Returns the datetime of display_since argument in MYSQL datetime format
      calculated according to the local time.
      :param display_since: =  all= no filtering
                              nd = n days ago
                              nw = n weeks ago
                              nm = n months ago
                              ny = n years ago
                              where n is a single digit number
      :return: string of wanted datetime.
              If 'all' given as argument, will return datetext_default
              datetext_default is defined in miscutils/lib/dateutils and
              equals 0000-00-00 00:00:00 => MySQL format
              If bad arguement given, will return datetext_default
              If library 'dateutil' is not found return datetext_default
              and register exception.
      """
      time_types = {'d': 0, 'w': 0, 'm': 0, 'y': 0}
      today = datetime.today()
      try:
          nb = int(display_since[:-1])
      except:
          return datetext_default
      if display_since in [None, 'all']:
          return datetext_default
      if str(display_since[-1]) in time_types:
          time_type = str(display_since[-1])
      else:
          return datetext_default
      # year
      if time_type == 'y':
          if (int(display_since[:-1]) > today.year -
                  1) or (int(display_since[:-1]) < 1):
              #   1 < nb years < 2008
              return datetext_default
          else:
              final_nb_year = today.year - nb
              yesterday = today.replace(year=final_nb_year)
      # month
      elif time_type == 'm':
          try:
              from dateutil.relativedelta import relativedelta
          except ImportError:
              # The dateutil library is only recommended: if not
              # available, then send warning about this.
              register_exception(alert_admin=True)
              return datetext_default
          # obtain only the date: yyyy-mm-dd
          date_today = datetime.now().date()
          final_date = date_today - relativedelta(months=nb)
          yesterday = today.replace(
              year=final_date.year,
              month=final_date.month,
              day=final_date.day)
      # week
      elif time_type == 'w':
          delta = timedelta(weeks=nb)
          yesterday = today - delta
      # day
      elif time_type == 'd':
          delta = timedelta(days=nb)
          yesterday = today - delta
      return yesterday.strftime("%Y-%m-%d %H:%M:%S")