You can add additional date formats in the tiddler $:/plugins/EventCal/cell/contents by adding [contains:cal-date<my-format>] to the <$list> filter (before the +[sort[]]) and defining my-format in a <$let> outside of the <$list>. You can use variables to put together your format as described in the ReadMe in the 3rd tab.
So, for ISO format, try putting [contains:cal-date<iso-date>] in the <$list> filter and then define iso-date like:
<$let iso-date={{{ [{!!cal-yy}][{!!cal-mm}][<date>]+[join[-]] }}} >
<$list of [contains:[]]...
...>
</$let>
and that should give you YYYY-MM-DD as the format. If you need times too you’ll need to define more variables and maybe join it in a different way.