Equivalent of std::mktime in LO

Hello All,

I have a patch here, https://gerrit.libreoffice.org/#/c/58152/ and I am
trying to replace

std::tm getDateTime(double nDateTime)
{
    long nDays = std::trunc(nDateTime);
    std::tm aDate = {};
    aDate.tm_year = 0;
    aDate.tm_mon  = 0;
    aDate.tm_mday = 0;
    aDate.tm_sec = getSecond(nDateTime);
    aDate.tm_min = getMinute(nDateTime);
    aDate.tm_hour = getHour(nDateTime);
    aDate.tm_wday = 0;
    aDate.tm_yday = 0;
    // Add number of days
    aDate.tm_mday += nDays;
    std::mktime(&aDate);
    return aDate;
}

with css::util:DateTIme. I am  having trouble finding out what is the
equivalent for std::mktime in LO.

Thanks,
Vikas Mahato