using words I'd say 12th of April ("April 12th" or "12th April" both work too since you won't mix up the 'name' of the month and the numeric day, if you were to carry this for number then "4th 12th" would be confusing

)
dd/mm/yy is used most places that aren't America btw

(or at least most countries I know of)
Also the universal way of doing it (and what I have to use for programming) is yyyy-mm-dd

[also I think you'll find you're doing it backwards, if you're talking historically

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