Add Comparing DateTime Structs as an elixir til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_774 TILs and counting..._
+_775 TILs and counting..._
 
 ---
 
@@ -123,6 +123,7 @@ _774 TILs and counting..._
 - [Assert An Exception Is Raised](elixir/assert-an-exception-is-raised.md)
 - [Binary Representation Of A String](elixir/binary-representation-of-a-string.md)
 - [Check For A Substring Match](elixir/check-for-a-substring-match.md)
+- [Comparing DateTime Structs](elixir/comparing-datetime-structs.md)
 - [Compute md5 Digest Of A String](elixir/compute-md5-digest-of-a-string.md)
 - [Counting Records With Ecto](elixir/counting-records-with-ecto.md)
 - [Create A Date With The Date Sigil](elixir/create-a-date-with-the-date-sigil.md)

elixir/comparing-datetime-structs.md

@@ -0,0 +1,26 @@
+# Comparing DateTime Structs
+
+> Remember, comparisons in Elixir using ==/2, >/2, </2 and friends are
+> structural and based on the DateTime struct fields. For proper comparison
+> between datetimes, use the compare/2 function.
+
+As the [DateTime docs](https://hexdocs.pm/elixir/DateTime.html) say, you'll
+want to use [`compare/2`](https://hexdocs.pm/elixir/DateTime.html#compare/2)
+in order to accurately compare two `DateTime` structs.
+
+```elixir
+{:ok, older} = DateTime.from_naive(~N[2016-05-24 13:26:08.003], "Etc/UTC")
+{:ok, newer} = DateTime.from_naive(~N[2017-11-24 13:26:08.003], "Etc/UTC")
+
+DateTime.compare(older, newer)
+#=> :lt
+
+DateTime.compare(newer, older)
+#=> :gt
+
+DateTime.compare(newer, newer)
+#=> :eq
+```
+
+When using `compare/2`, you'll get one of `:lt`, `:gt`, or `:eq` as a
+result, meaning _less than_, _greater than_, or _equal_ respectively.