I think it was 31 March... I've doubted my own basic maths and have literally counted the days on the calendar... Whichever way you do it, day 15 was yesterday! Day 1 being 17 March.
No, the NT 10-K specifies that the filing must be made "on or before the fifteenth calendar day following the prescribed due date," and the prescribed due date was March 16. Fifteen calendar days after that is March 31.