In the UK, and possibly other English speaking countries, these formats are fairly obscure. I would never naturally write a date like that. The day would always go first, like 11th Feb 2018.
I would imagine that across the many languages Discourse is used in, these discrepancies get far harder to cope with.
I agree with @mcwumbly that a date picker would be a good option. I would imagine it is also waaaaaay easier to implement.
If it's a magic feature, then it needs to support a multitude of formats to allow users to discover it naturally. A date picker is nice and clear, with no confusion (albeit at the expense of some screen real-estate).