OK, ignore this, worked it out almost immediately after I posted (having been trying for half a day)
Need to get the NotesDateTime value first, so:
var ddt:NotesDateTime = session.createDateTime(getComponent("DateField").getValue());
var dd = new Date(ddt.toJavaDate())
I need a js date as I then need to get the day and month values (which I don't think you can do in SSJS from a notesdatetime)