Looks like that author prefixes the month with "M". Dunno why.

Sometimes DD-MM-YYYY and MM-DD-YYYY can be confused, such that if one of those formats is used, it could be helpful to clarify. However, YYYY-MM-DD is typically pretty unambiguous, so it seems odd here.

I guess it's probably meant in analogy to, e.g., "2020-Q1" (which would be the first Quarter of 2020) or "2020-H1" (which would be the first Half of 2020).

I do it because it's visually unambiguous. It is clear what is the day number, what is the month number and what is the year number. I have no idea where Apple got this, but it's the date format used in the iOS Lojban locale. I kinda got used to it because it's also faster for me to type.

Year 2020, month 2 (February), day 10 (the 10th). In RFC-3339, 2020-02-10.

Year 2020, Month 2, Day 10

If the day is a single digits do you prepend D to it? Eg October 2nd?