qv4dateobject.cpp uses incorrect daylight savings calculation

// This implementation fails to take account of past changes in standard offset.
static inline double DaylightSavingTA(double t, double /*localTZA*/)
{
    struct tm tmtm;
#if defined(Q_CC_MSVC)
    __time64_t  tt = (__time64_t)(t / msPerSecond);
    // _localtime_64_s returns non-zero on failure
    if (_localtime64_s(&tmtm, &tt) != 0)
#elif !defined(QT_NO_THREAD) && defined(_POSIX_THREAD_SAFE_FUNCTIONS)
    long int tt = (long int)(t / msPerSecond);
    if (!localtime_r((const time_t*) &tt, &tmtm))
#else
    // Returns shared static data which may be overwritten at any time
    // (for MinGW/Windows localtime is luckily thread safe)
    long int tt = (long int)(t / msPerSecond);
    if (struct tm *tmtm_p = localtime((const time_t*) &tt))
        tmtm = *tmtm_p;
    else
#endif
        return 0;
    return (tmtm.tm_isdst > 0) ? msPerHour : 0;
}