How to decode a week code (ISO week date)

Question

Is there a way on how to decode a week code or ISO week date using CTime ? I am currently creating an expiration date that is based on week code. I'm having a hard time adding the CTime and CTimeSpan (shelf_life) because the output is not accurate.

If the shelf_life is 6 months, my output is accurate. But when the shelf_life is more than a year, the output is 4 months overdue.

This is my code:

CTime tt, t11;
CString Cweek, Cyear;
int yy, mm, wknum, yrnum;

tt = CTime::GetCurrentTime();
yy = tt.GetYear();
mm = tt.GetMonth();

wknum = atoi(tt.Format("%U"));
yrnum = atoi(tt.Format("%y"));
t11 = CTime(yy, mm, 1, 0, 0, 0);
if (t11.GetDayOfWeek() == 4)
    wknum = wknum ++;

Cweek.Format("%02d", wknum);
Cyear.Format("%02d", yrnum);

CTimeSpan shelf_no = CTimeSpan(atoi(view->m_pODBCPartsNo->m_shelf_life), 0, 0, 0);
CTime expiration_date = tt + shelf_no;