The way to write 21 is to change 20 to 1, 21st. So the abbreviation is 21st.

22nd.

23rd.

For example:

September 13th should be 13th, September, pronounced the thirteen of September.

September 12 should be 12th, September, pronounced the twelfth of September.

The changing rules of cardinal numerals are as follows:

1. There is no rule to follow from 1 to 10: one, two, three, four, five, six, seven, eight, nine, ten;

2. 11 to 19: eleven, twelve, thirteen, fourteen, fifth, sixteen, seventeen, eighteen, nineteen;

3. From 20 to 99: the number of integers In addition to twenty (twenty), thirty (thirty), forty (forty), fifty (fifty), eighty (eighty), which are special forms, sixty (sixty), seventy (seventy), ninety (Ninety) is formed by adding the suffix -ty after its single digit form.

4. Add "hundred" to the base numeral form of the hundreds digit, which means hundreds, and add "and" between the tens and hundreds.