It doesn't make sense mathematically because it isn't a mathematical notation, it's linguistic in nature. In Europe, '4th May' is shortened to 4/5. In the US,

It doesn't make sense mathematically because it isn't a mathematical notation, it's linguistic in nature. In Europe, '4th May' is shortened to 4/5. In the US, 'May 4th' is shortened to 5/4. Neither is right, wrong or illogical. I'm from the UK, but have spent a lot of time working with US based clients/colleagues. They only way I've found to avoid confusion and ambiguity is to use YYYY/MM/DD.