If you use MFC then the simplest way to add days is to use COleDateTime() class for the dates from the DateTime picker together with COleDateTimeSpan class to add/subtract the days:
CDateTimeCtrl *pDt = (CDateTimeCtrl*)GetDlgItem(IDC_DATEPICKER1);
COleDateTime dt1;
pDt->GetTime(dt1);
COleDateTimeSpan Day(1, 0, 0, 0);
COleDateTimeSpan Days2(2, 0, 0, 0);
COleDateTime tomorrow = dt1 + Day;
COleDateTime beforeYesterday = dt1 - Days2;