American English (US)
6-10-2021 = June 10th, 2021:
Month is said first in American English.
Format: Month day, year.
June the tenth, twenty twenty-one
or: June the tenth, two thousand twenty-one
* The definite article "the" can be left out.
June tenth, twenty twenty-one
or: June tenth, two thousand twenty-one
British English (UK)
10-6-2021 = 10th June, 2021:
Day is said first in British English.
Format: day Month, year.
the tenth of June, twenty twenty-one
or: the tenth of June, two thousand and twenty-one
The date. Note.
In (US) American English date representation, usually the month comes before the day, then followed by year.
This is called middle endian representation: the middle significant value, the month, comes first. The day is the least significant value and the year is the most significant value in the sequence.
In (UK) British English, as in the most of Europe, usually the day comes before the month, then followed by year.
This is called little endian representation: the least significant value, the day, comes first. The month has middle significance and the year is the most significant.
Big-endian is a term which is related to an order in which the most significant value in the sequence comes first, so, in our case, the year comes first, usually followed by month and then by day.
Notes on Writing Out Numbers and Years:
1: It's correct to hyphenate all compound numbers from 'twenty-one' (21) through 'ninety-nine' (99). The hyphen is the minus sign, as in 'thirty-four' (34).
2: In American English, unlike British English, when writing out years of three or more digits, the word 'and' is not used after 'hundred' or 'thousand'; i.e. 963 is written out as 'nine sixty-three' (American English) and not as 'nine hundred and sixty-three' (British English); 2025 is 'two thousand twenty-five' (American English) and not 'two thousand and twenty-five' (British English).
3. Up to 1999 the years are spoken as hundreds, but for shortening, the words 'hundred and' are omitted: i. e. 1999 = 'nineteen (hundred and) ninety-nine' = 'nineteen ninety-nine'.
4. After the year 2000 the years are spoken as normal numbers, but for shortening, in colloquial language, the years are pronounced in groups of two: i. e. 2000 = 'two thousand'; 2001 = 'two thousand (and) one', but also 'twenty oh one'.
5. Do not use commas when writing out in words numbers above 999; i.e. 1234 is "twelve thirty-four" and not "twelve, thirty-four".
6. Do not use commas when writing in digits years above 999; i.e. it is 1234; 1973, 2021, and not 1,234; 1,973, 2,021.