Incorrect spelling, explanation: this form is an incorrect one, because to add suffix -rd in order to create an ordinal number, we need the word to end with three (for example 43, 983, 48583 etc.). The word thirty doesn’t have three in it so the suffix for ordinal number is regular: -th. Therefore the correct form is 30th, not 30rd.

Correct spelling, explanation: there are several suffixes used to create ordinal numbers: -st, -nd, -rd and -th. First three of them are reserved respectively to numbers one, two and three (and all numbers which include these at the end, like 51, 62, 93 etc.) and suffix -th is used to any other number. For this reason, the correct ordinal form of number 30 is 30th and not 30rd, since there is no three in the word thirty.

Definition of 30th:
numeral, ordinal number between 29th and 31st
Jack was very successful being 30th in this international programming contest.
Paris is 30th city Mary have lived in. That’s a lot of places to live!

Incorrect spelling

Incorrect spelling