に当たる【にあたる】

に当たる says that one thing corresponds to another in a different system: a relative in a family, a word in another language, an amount in another unit, a date in a calendar. It follows a noun, and the thing being placed takes は or が.

For a date, English says "falls on".

Counting as

After a word for a category of behaviour, に当たる says an act counts as that: rude, illegal, a violation. The act is usually a clause with の or こと.

In the negative, にはあたらない当たらない says a reaction is not called for: "there is no need to".

に当たる and にあたって

にあたって is a separate pattern meaning "on the occasion of", for the start of something important.