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).

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)