I'm often asked something like "What's the date separator for locale XXX?" Well, not every locale fits our preconceived MM/dd/yyyy concepts. Some of the built-in values that developers may not expect:
And since there are user overrides and custom locales, this just scratches the surface. (Don't ignore user overrides!) Some obvious ones:
So the moral is to use the short (or long) date formats provided by the system and don't try to build your own. That's a bit tricky in some places, like Calendar titles, or controls to filter dates, but it's important to respect the user settings and not make assumptions about their date (or time) formats.
Short time formats have similar issues. (eg: "12h 42m 15s", or "12:42 uhr", or....). Long dates are even more varied, with prepositions in the format, or even different word forms for the month names, etc.