#include <sys/utime.h>

...
QDateTime dt;
...

#ifdef Q_OS_WIN
    filePath.replace("/", "\\");
#endif    

struct utimbuf times;
times.actime = times.modtime = dt.toTime_t();     
utime(filePath.toLatin1().data(), &times);

#ifdef Q_OS_WIN
#include <qt_windows.h>
#endif 

...
 QDateTime dt;
 SYSTEMTIME systemTime;
 FILETIME fileTime, localFileTime;
 HANDLE hFile;
...

#ifdef Q_OS_WIN
    systemTime.wYear = dt.date().year(); 
    systemTime.wMonth = dt.date().month(); 
    systemTime.wDayOfWeek = dt.date().dayOfWeek(); 
    systemTime.wDay = dt.date().day(); 
    systemTime.wHour = dt.time().hour(); 
    systemTime.wMinute = dt.time().minute(); 
    systemTime.wSecond = dt.time().second(); 
    systemTime.wMilliseconds = dt.time().msec(); 
    SystemTimeToFileTime(&systemTime, &localFileTime);
    LocalFileTimeToFileTime(&localFileTime, &fileTime);
    filePath.replace("/", "\\");
    hFile = CreateFile(filePath.utf16(), GENERIC_WRITE, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);    
    SetFileTime(hFile, &fileTime, &fileTime, &fileTime);
    CloseHandle(hFile);
#endif