You Need to typecast pNMHDR to LPNMDATETIMECHANGE.Refer MSDN for
NMDATETIMECHANGE
Sample code:
LPNMDATETIMECHANGE pDTChange = reinterpret_cast<lpnmdatetimechange>(pNMHDR);
CString strDDMMYYY;
strDDMMYYY.Format("DD::MM::YYYY %02d::%02d::%04d", pDTChange->st.wDay, pDTChange->st.wMonth, pDTChange->st.wYear);
</lpnmdatetimechange>