Two time intervals are expressed in milliseconds.

The following is the msdn prompt:
Remarks
It is not recommended that you add and subtract values from the systemtime structure to obtain relative times. Instead, you shoshould

1. Convert the systemtime structure to a filetime structure.
2. Copy the resulting filetime structure to a large_integer structure.
3. Use normal 64-bit arithmetic on the large_integer value.

So you can do this:
Filetime ftstart, ftend;
Systemtimetofiletime (& m_ststart, & ftstart );
Systemtimetofiletime (& m_stend, & ftend );

Large_integer listart, liend;
Listart. highpart = ftstart. dwhighdatetime;
Listart. lowpart = ftstart. dwlowdatetime;
Liend. highpart = ftend. dwhighdatetime;
Liend. lowpart = ftend. dwlowdatetime;

_ Int64 llusedtime = (liend. quadpart-listart. quadpart)/10000; // you want time