It's important to be consistent across all of WikiTree, and not have people doing things one way for some sailings and a different way for others. The established pattern for emigrant/immigrant passages (not just to Australia, but everywhere) is to date the sailings by departure, to use a full date, and to use the American style of dating. (As in, "May 13, 1787", rather than "13 May 1787".) Personally, I would have done the some things differently (like using a date format of "17870513", so that, as ships got fast enough to do multiple sailings per year, the trips would automatically sort chronologically), but even the worst system imaginable would be better than dozens of incompatible systems, with everybody doing their own thing.