Steve, it seems to me that for MSVC the EINVAL test should come first:
   
    _Py_BEGIN_SUPPRESS_IPH
    olderr = errno;
    errno = 0;
    buflen = format_time(outbuf, i, fmt, &buf);
    err = errno;
    errno = olderr;
    _Py_END_SUPPRESS_IPH
    
    if (buflen == 0 && err == EINVAL) {
        PyErr_SetString(PyExc_ValueError, "Invalid format string");
        break;
    }

Then the old test could be restored, i.e. (buflen > 0 || i >= 256 * fmtlen).