Define the following prototype in the header file

JNIEXPORT void JNICALL Java_JavaHowTo_setSystemTime
  (JNIEnv *, jobject, jshort, jshort);

the JNI function

JNIEXPORT void JNICALL Java_JavaHowTo_setSystemTime
  (JNIEnv *env, jobject obj, jshort hour, jshort minutes) {
    
    SYSTEMTIME st;

    GetLocalTime(&st);  
    st.wHour = hour;      
    st.wMinute = minutes;  
    SetLocalTime(&st);   
}

The Java JNI wrapper would be

 class JavaHowTo {
    
    public native void setSystemTime( short hour, short minutes);
    
    static {
        System.loadLibrary("javahowto");
    }
}

And finally, to use it

 public class JNIJavaHowTo {
    
    public static void main(String[] args) {
        
        short hour = 10;
        short minutes = 21;
        
        // this example will set the system at 10h21 using the Windows API
        // SetLocalTime.
        
        JavaHowTo jht = new JavaHowTo();
        // set the time at 10h21
        jht.setSystemTime(hour, minutes);
    }
}