If you use
@timestamp(time) // this class member contains the timestamp
in a declare, it informs the compiler that your field called time contains the time. It does not imply that the Engine should set the time - you'll have to do this prior to the insert. You may, of course, use the pseudo-clock to obtain a suitable time.
Only by omitting this from the declare you'll achieve the automatic setting of the timestamp in a field managed entirely by Drools.