AFAIK, year/month/day is not a format than anyone uses. It seems more
sensible to me to use ISO 8601, as that's standard and unambiguously
year-month-day.
Compatibility-wise, I think it's acceptable to change the default
format. It's most for human consumption. It's plausible that a few
tools parse this format, but it has already changed in the past (for
instance to add the current revision).
This comment should be updated to reflect the newly expected format