Add documentation about measuring time

src/pages/core/_meta.json

@@ -3,5 +3,6 @@
   "entrypoints": "Entrypoints",
   "architecture": "Architecture",
   "standard-library": "Standard Library",
+  "advanced": "Advanced",
   "specification": "Specification"
 }

src/pages/core/advanced.mdx

@@ -0,0 +1,17 @@
+---
+tags: ["core", "advanced"]
+---
+
+import { Callout } from "nextra/components";
+
+# Advanced topics
+
+In this part of the documentation, we go over some more "advanced" things you
+can do with the functionality CosmWasm core offers.
+
+<Callout>
+  While the name of this section is "advanced", this doesn't mean that we want
+  to imply that you only need these functions in advanced contexts. The wording
+  "advanced" moreso refers to the fact that you should be familiar with
+  endpoints, the structs that are passed to the endpoints, etc.
+</Callout>

src/pages/core/advanced/_meta.json

@@ -0,0 +1,3 @@
+{
+  "measuring-time": "Measuring time"
+}

src/pages/core/advanced/measuring-time.mdx

@@ -0,0 +1,31 @@
+---
+tags: ["core", "advanced"]
+---
+
+import { Callout } from "nextra/components";
+
+# Measuring time
+
+Accessing the current time is useful in a lot of different contexts but time is
+hard. It is hard to coordinate, hard to keep in sync, and this gets worse in
+distributed settings such as blockchains.
+
+In CosmWasm, we solve this by passing you some information about the blockchain
+you are running on whenever an entrypoint is invoked.
+
+In each of the entrypoints, you get a parameter of the type `Env` and this
+struct contains the field `block`. The struct contained in this field has a
+bunch of different information about the current state of the blockchain you are
+running on.
+
+> The documentation about the `BlockInfo` struct can be
+> [found here](https://docs.rs/cosmwasm-std/latest/cosmwasm_std/struct.BlockInfo.html)
+
+The timestamp contained in this struct can be safely used in your program as the
+source of the current time. Well, kinda. It won't be 100% matching the current
+time, as it is the timestamp of the block we are currently operating on.
+
+But you can rely on this timestamp to have the following properties:
+
+- Consistent across the chain (every validator on the chain has the same info)
+- Monotonic (it will ever only increase or stay the same; never decrease)