Are you asking about how to write times? As in half past seven? In standard British English, the convention is to write 7.30; in American English, the convention is 7:30. As for the IELTS exam, it doesn't matter which you use. Both are equally correct, and both are acceptable. Even if you were to write something completely non-standard (such as a comma), you might still get your mark. On many parts of the IELTS exam - such as the gap-fill questions on the reading paper - markers ignore punctuation. Spelling - either British or American - has to be correct, but there's a lot of leeway when it comes to punctuation.