A diary and a calendar are different things. In a diary you typically have a good bit of space to note down things that are happening/happened that day (at least this is the typical meaning in the UK). This is less true of calendars (particularly digital ones).

In the US we commonly call both of these "calendars". If we need to distinguish between the two, we'll generally use "day planner" to refer to the book you're calling a diary.

Huh? It's a "journal".

I suspect we're encountering a planning vs logging fork here. If you primarily use a daily page to log or track what you did, then terms like journal or diary seem most natural to you. If you primarily use it to plan or list what you need to do in the future, then you're more likely to call it a day planner.

HN seems more highly populated with loggers and trackers compared to the general public. A quick search on Amazon US for "day planner", "journal", and "diary" suggests most people associate journal and diary with free-form pages and day planner with date-based pages (implying most people who use date-based pages are more planners than loggers)

To me "journal" is something 15-year olds will say in order to sound adult, and adults just call it a "diary". I guess I need to update this view.