[elzbardico]

A good heuristic is that if you are outside US, assume DD-MM-YYYY.

[sampo]

We don't need to assume, there is "2021-05-10" in the bottom of the article.

[SAI_Peregrinus]

To be pedantic that still requires assuming the author is using a sane convention (RFC3339) or convention including sane formats (ISO 8601) and not something ridiculous like %V-%W-%O or %Y-%D-%M. Thankfully the sane convention seems highly likely.