Ah, I'm not talking about Ruby, I'm talking about language design in general.

I'm currently mostly doing Rust, so I can only really name that as an example (even though there's very likely other languages that allow this, too), but yeah, here's for example the 64-bit signed integer in Rust: https://doc.rust-lang.org/stable/std/primitive.i64.html

That is a primitive type, not an object, so it is exactly 64-bits in size and stored on the stack, not the heap. But as you can see in the documentation, Rust allows for associated functions anyways, like for example:

let my_number: i64 = -3;
my_number.abs()  //returns the absolute value, so 3

That's because that last call is just syntactic sugar for calling the same function statically on the type:

i64::abs(my_number)

days is a method on the Numeric class in Rails, and it creates an instance of ActiveSupport::Duration with self passed to the constructor (this is a bit of a simplification, because it actually calls the class method days on Duration which converts the number of days into seconds before creating the Duration instance).