Database Suggestion

Could you explain what you would do on your server and what you would do on device (or in iCloud) ?

Why store the user calendar on your server ? To let others share ?

On the server I would store analytics about the user such as performance, suggestions about the usage of the calendar, profile picture, login, menu settings such as layout of the app, text fields for the todo and calendar descriptions with images and attatchments.
I'm trying to think of a use of Core Data but right now I don't think I'll be using it.
Eventually I would also let the users share the calendar yes, but the main function I think that I want it on my server is because of cross plattaform, because what if the user is on Windows and I support all apple devices but not windows? The if I create a web and this person wants to use it on the web but doesn't have apple, he/she would be in trouble.
So I'm trying to decide what would be the best approach for all this for cross platform.

I did not see the spec, so my comments are to be taken with a pinch salt.

I would look in the direction of a SQL database that you interface with JSON requests.

First, I really appreciate your input on this, it's very important to me.

What do you mean specs, I can provide it. This project will be a long term project so I want to make it right and scalable from the beginning.

Also sql, which one, MySQL? Any recomendations on where to store it, for example AWS?