Incrementing a date value

I am trying to get the system time, increment it by x number of days, and use incremented date to set a field in the database. Here is what I have so far:
SYSTEMTIME st; GetSystemTime(&st);
FILETIME ft;
SystemTimeToFileTime(&st,&ft);
LARGE_INTEGER li = {ft.dwLowDateTime,ft.dwHighDateTime};
<** WHAT DO I DO WITH THE LARGE INTEGER TO ADD
X DAYS???***>
ft.dwHighDateTime = li;
FileTimeToSystemTime(&ft,&st);

I have tried shifting and adding, but don't know anything about 64-bit arithmetic or operations.

Please Help!

Therese