mfc、C++求时间差

CString CommonClass::DiffTime(CString time1,CString time2)
{
/*10:10:01-09:00:00*/
CString Sum = "",tem;

int h, m,s,h1,m1,s1;

sscanf(time1,"%02d:%02d:%02d", &h,&m,&s);

sscanf(time2,"%02d:%02d:%02d",&h1,&m1,&s1);
if (h < h1||h >= 12)
{
return "";
}
if (h == h1)
{
m = m - m1;

if (m != 0)
{
tem.Format("%d分",m);
Sum = tem;
return Sum;

}
else
{
return "";
}
}