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.
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).