在DateTimeField（Python 3.8）的源代码中，它具有以下错误处理：

default_error_messages = {
        'invalid': _('“%(value)s” value has an invalid format. It must be in '
                     'YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ] format.'),
        'invalid_date': _("“%(value)s” value has the correct format "
                          "(YYYY-MM-DD) but it is an invalid date."),
        'invalid_datetime': _('“%(value)s” value has the correct format '
                              '(YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ]) '
                              'but it is an invalid date/time.'),
    }

uuuuu是什么意思