Add Rhai book.

doc/src/language/timestamps.md

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+`timestamp`'s
+=============
+
+{{#include ../links.md}}
+
+Timestamps are provided by the [`BasicTimePackage`](/rust/packages.md) (excluded if using a [raw `Engine`])
+via the `timestamp` function.
+
+Timestamps are not available under [`no_std`].
+
+The Rust type of a timestamp is `std::time::Instant` ([`instant::Instant`](https://crates.io/crates/instant) in [WASM] builds).
+
+[`type_of()`] a timestamp returns `"timestamp"`.
+
+
+Built-in Functions
+-----------------
+
+The following methods (defined in the [`BasicTimePackage`](/rust/packages.md) but excluded if using a [raw `Engine`]) operate on timestamps:
+
+| Function                      | Parameter(s)                       | Description                                              |
+| ----------------------------- | ---------------------------------- | -------------------------------------------------------- |
+| `elapsed` method and property | _none_                             | returns the number of seconds since the timestamp        |
+| `-` operator                  | later timestamp, earlier timestamp | returns the number of seconds between the two timestamps |
+
+
+Examples
+--------
+
+```rust
+let now = timestamp();
+
+// Do some lengthy operation...
+
+if now.elapsed > 30.0 {
+    print("takes too long (over 30 seconds)!")
+}
+```