I've seen people do YYYY/DD/MM, so your friend's format isn't foolproof. OTOH although I'm sure some insane person somewhere has done YYYY-DD-MM I've never seen it so YYYY-MM-DD should be safe.
Some languages pronounce the date as year, day, month so some people write YYYY/DD/MM. Since both YYYY/DD/MM and YYYY/MM/DD make sense to different people, neither should be used.
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I've seen people do YYYY/DD/MM, so your friend's format isn't foolproof. OTOH although I'm sure some insane person somewhere has done YYYY-DD-MM I've never seen it so YYYY-MM-DD should be safe.
YYYY/MM/DD is in order of size. Y > M > D makes some kind of sense.
Some languages pronounce the date as year, day, month so some people write YYYY/DD/MM. Since both YYYY/DD/MM and YYYY/MM/DD make sense to different people, neither should be used.