Call IRunningObjectTable::NoteChangeTime

Whenever you make a change to Cosmo's Polyline figure, it calls CPolylineAdviseSink::OnDataChange in DOCUMENT.CPP. This sets the dirty flag in the document by calling CCosmoDoc::FDirtySet, which in turn calls CFigure::SendAdvise(OBJECTCODE_DATACHANGED). We send IAdviseSink::OnDataChange notifications as necessary, but we also call IRunningObjectTable::NoteChangeTime:

case OBJECTCODE_DATACHANGED:
    if (NULL!=m_pIDataAdviseHolder)
        {
        m_pIDataAdviseHolder->SendOnDataChange
            (m_pImpIDataObject, 0, 0);
        }

    if (0!=m_dwRegROT)
        INOLE_NoteChangeTime(m_dwRegROT, NULL, NULL);

    break;

The helper function INOLE_NoteChangeTime takes the registration key from IRunningObjectTable::Register and calls the running object table for us. Its other two arguments are a FILETIME structure and a filename. If we pass NULL for the FILETIME, INOLE_NoteChangeTime will use the current time or try to obtain it from the filename:

STDAPI_(void) INOLE_NoteChangeTime(DWORD dwReg, FILETIME *pft
    , LPTSTR pszFile)
    {
    IRunningObjectTable    *pROT;
    FILETIME                ft;

    if (NULL==pft)
        {
        CoFileTimeNow(&ft);
        pft=&ft;
        }

    if (NULL!=pszFile)
        GetFileTimes(pszFile, pft);

    if (FAILED(GetRunningObjectTable(0, &pROT)))
        return;

    pROT->NoteChangeTime(dwReg, pft);
    pROT->Release();
    return;
    }

Using the current time is good enough for Cosmo.