Adding info to the arithmetic section!

src/api/arithmetic.md

@@ -1 +1,63 @@
 # Arithmetic
+
+Let's suppose you have an ecommerce and you want to show your users the date of arrival of their purchases.
+
+In order to know the arrival date, you would get the today's date and add the amount of time that the delivering process takes. That's sounds possible, but a little verbose for you to write it yourself. Fortunately, the `time` crate has you covered. Let's jump into that!
+
+First, we'll need to import the `OffsetDateTime` struct that has a implementation called `now_utc()` that we can use for discover the current date of our user purchase.
+
+```rust
+use time::{OffsetDateTime, Date};
+
+fn get_arrival_date() {
+
+    let current_date: Date = OffsetDateTime::now_utc().date();
+
+}
+```
+
+The `OffsetDateTime::now_utc()` returns a `OffsetDateTime` value like this one: `2024-04-06 13:26:19.8381466 +00:00:00`. As we are more interested in the date itself, we use the `.date()` method to get a `Date` struct that we have to import as well. So now our `current date` variable is like this: `2024-04-06`.
+
+Now let's add our estimated time to deliver to that current date. We'll use a method from the `Date` struct called `.checked_add()`. It receives a value of the `Duration` struct measured in seconds and adds to our current date. Let's calculate our a deliver time using the `Duration` struct from `time` first:
+
+```rust
+
+use time::Duration;
+
+let time_to_deliver: Duration  = 45*(Duration::DAY);
+
+
+```
+
+Considering that we take 45 days to deliver the product(not a very efficient service, I know) we can use the `Duration::DAY` constant that will give the amount of seconds of a day. This helps us to avoid calculting the amount of seconds for ourselves and make our code clear to others as well.
+
+As we take 45 days to deliver, we multiply the amount of seconds returned by 45. Now the `time_to_deliver` variable is a `Duration` that represents our delivering service. Excactly what we need to use the `.checked_add()` method!
+
+```rust
+
+let date_of_arrival:Date = current_date.checked_add(time_to_deliver).expect("Something went wrong");
+
+
+```
+
+Finally, we use the `.checked_add()` method from our `current_date` variable passing the `time_to_deliver` as a parameter. As this return an `Option<T>` enum, we use `.expect()` to handle when the value returned is `None`. If you are not familiar with the `Option` enum, here is a [link from the Rust Book](https://doc.rust-lang.org/book/ch06-01-defining-an-enum.html?highlight=optio#the-option-enum-and-its-advantages-over-null-values) where you can learn more about it!
+
+```rust
+
+use time::{Duration, OffsetDateTime, Date};
+
+fn main() {
+
+    let current_date = OffsetDateTime::now_utc().date();
+
+    let time_to_deliver: Duration  = 48*(Duration::DAY);
+
+    let date_of_arrival:Date = current_date.checked_add(time_to_deliver).expect("Something went wrong");
+
+
+    println!("Your package will arrive in {}!", date_of_arrival);
+}
+
+```
+
+And now our user know when the package will arrive :)

src/api/conversion-between-types.md

@@ -1 +1,2 @@
 # Conversion between types
+