docs: def _calculate_offset(date, local_tz):
      """
      input :
      date : date type
      local_tz : if true, use system timezone, otherwise return 0
  
      return the date of UTC offset.
      If date does not have any timezone info, we use local timezone,
      otherwise return 0
      """
      if local_tz:
          #handle year before 1970 most sytem there is no timezone information before 1970.
          if date.year < 1970:
              # Use 1972 because 1970 doesn't have a leap day
              t = time.mktime(date.replace(year=1972).timetuple)
          else:
              t = time.mktime(date.timetuple())
  
          # handle daylightsaving, if daylightsaving use altzone, otherwise use timezone
          if time.localtime(t).tm_isdst:
              return -time.altzone
          else:
              return -time.timezone
      else:
          return 0