The TDUSRTIMER system variable does not store the elapsed milliseconds, but instead stores the time elapsed form the last reset in a Julian date format. The easiest way to get the time elapsed in seconds is to use the AcDbDate class.
void ASDKgetTime()
{
struct resbuf rUsrTime;
acedGetVar(_T("TDUSRTIMER"), &rUsrTime);
AcDbDate testDate ;
testDate.setJulianFraction(rUsrTime.resval.rreal) ;
acutPrintf (
_T("\nMinute: %d\nSec: %d\nMilisec: %d"),
testDate.minute (),
testDate.second (),
testDate.millisecond ()) ;
}