groovy:000> cal = Calendar.getInstance();
===> java.util.GregorianCalendar
groovy:000> cal.set(2016,Calendar.SEPTEMBER,12);
===> null
groovy:000> cal.add(Calendar.DAY_OF_YEAR, 120);
===> null
groovy:000> cal.getTime();
===> Tue Jan 10 01:23:10 AEDT 2017

May be OpenText guys have changed their mind??

UPD.

There is a hypothesis about January 13: