Don't even have to go that far, outside of white-collar jobs and some groups weirdly obsessed with scheduling, most people don't use calendars at all, but their manager/boss/significant-other does that for them :)

Have never seen 27-7, as someone in rest-of-world

This is so confidently wrong it's funny. In Australia dd/mm/yy is the default.

I've never seen dd-mm-yy. It's usually dd.mm.yy, dd.mm.yyyy, or yyyy-mm-dd, with some dd/mm/yy sprinkled in for general confusion.

Or 27.7. in some other places.

Nope,

There are quite some countries around the world using d/m/y

https://en.wikipedia.org/wiki/List_of_date_formats_by_countr...

Algeria, Belgium, Brazil, Chile...

Not really. Spain's traditional format is dd/mm/yyyy with slashes. This applies for a good chunk of Europe. Germany/Austria uses dot, I think nordic countries embraced the dash. While you might see more adoption in offical/digital contexts, I just double checked a few popular spanish websites, all slashes.

27–7 would be 20.

You mean 27.7