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To be fair, proper ISO 8601 specifies hyphens as the separator between date elements, and I don't think I've ever seen a XXXX-XX-XX (with hyphens) be used for YYYY-DD-MM. Just XX-XX could perhaps be ambiguous, but fortunately that's not allowed by the standard, and anyone using just year-day for XXXX-XX is absolutely trolling. YYYY-DDD could have a use, though should really use a separate separator to not sort together IMO. A year-week designation could possibly look like XXXX-XX, but that seems unlikely to just be dropped in that format without context, at least to my western US sensibilities.