code: def _tzstr(self, sep=":"):
          
          off = self.utcoffset()
          if off is not None:
              if off.days < 0:
                  sign = "-"
                  off = -off
              else:
                  sign = "+"
              hh, mm = divmod(off, timedelta(hours=1))
              assert not mm % timedelta(minutes=1), "whole minute"
              mm //= timedelta(minutes=1)
              assert 0 <= hh < 24
              off = "%s%02d%s%02d" % (sign, hh, sep, mm)
          return off
docs: Return formatted timezone offset (+xx:xx) or None.