There is an exmple about change start time/date , it use the realtime. You can

customize the startTime as you wish.

The Code need to be added to OnRunStart Trigger (Tools -> Modeling Logic -> Model Trigger -> On Run Start).

double startTime = applicationcommand("getunixtime");
treenode startNode = getmodelunit(START_TIME_NODE);
set(startNode, startTime);
applicationcommand("convertunixtime", startNode, startTime);
treenode stopTime = getmodelunit(STOP_TIME_NODE);
set(stopTime, startTime + getmodelunit(TIME_MULTIPLE) * get(node("MAIN:/project/exec/stoptime")));
applicationcommand("convertunixtime", stopTime, get(stopTime));
set(getmodelunit(CURRENT_TIME_NODE), 0); //Cause the RunPanel to update