Calculators (and related mental activities) as hobby or as avocation?

[David Hayden]

A hobby is something that you enjoy doing in your spare time.

A passion is something that you really enjoy doing. The connotation is that you'd do it full time if you were independently wealthy or could make a living from it.

A job or vocation is something you do that pays the bills.

"Day job" is slang for a job that you don't like.

"My day job is X but my passion is Y." This means the person would do Y full time but it doesn't pay well, so they do X which pays better, but doesn't interest them.