Add site.feed.updated = latest_post setting

README.md

@@ -45,6 +45,17 @@ feed:
 
 To note, you shouldn't have to do this unless you already have a feed you're using, and you can't or wish not to redirect existing subscribers.
 
+### Updated timestamp
+
+By default, the feed will set `<updated>` to the current time at build time, as provided by Jekyll via `site.time`.
+
+Enable the `updated: latest_post` setting to use a deterministic value based on when the latest post was published or updated. (The [jekyll-last-modified-at](https://github.com/gjtorikian/jekyll-last-modified-at) plugin is supported to help provide post modification times).
+
+```yml
+feed:
+  updated: latest_post
+```
+
 ### Optional front matter
 
 The plugin will use the following post metadata, automatically generated by Jekyll, which you can override via a post's YAML front matter:

lib/jekyll-feed/feed.xml

@@ -6,7 +6,6 @@
   <generator uri="https://jekyllrb.com/" version="{{ jekyll.version }}">Jekyll</generator>
   <link href="{{ page.url | absolute_url }}" rel="self" type="application/atom+xml" />
   <link href="{{ '/' | absolute_url }}" rel="alternate" type="text/html" {% if site.lang %}hreflang="{{ site.lang }}" {% endif %}/>
-  <updated>{{ site.time | date_to_xmlschema }}</updated>
   <id>{{ page.url | absolute_url | xml_escape }}</id>
 
   {% assign title = site.title | default: site.name %}
@@ -51,7 +50,20 @@
     {% assign posts = posts | where_exp: "post", "post.draft != true" %}
   {% endunless %}
   {% assign posts = posts | sort: "date" | reverse %}
+
   {% assign posts_limit = site.feed.posts_limit | default: 10 %}
+
+  {% assign last_updated = nil %}
+  {% if site.feed.updated == "latest_post" %}
+    {% for post in posts limit: posts_limit %}
+      {% assign post_updated = post.last_modified_at | default: post.date %}
+      {% if last_updated == nil or post_updated > last_updated %}
+        {% assign last_updated = post_updated %}
+      {% endif %}
+    {% endfor %}
+  {% endif %}
+  <updated>{{ last_updated | default: site.time | date_to_xmlschema }}</updated>
+
   {% for post in posts limit: posts_limit %}
     <entry{% if post.lang %}{{" "}}xml:lang="{{ post.lang }}"{% endif %}>
       {% assign post_title = post.title | smartify | strip_html | normalize_whitespace | xml_escape %}

spec/jekyll-feed_spec.rb

@@ -118,6 +118,7 @@
 
   context "parsing" do
     let(:feed) { RSS::Parser.parse(contents) }
+    let(:overrides) { { "time" => Time.parse("2018-12-31") } }
 
     it "outputs an RSS feed" do
       expect(feed.feed_type).to eql("atom")
@@ -136,6 +137,24 @@
       expect(feed.generator.version).to eql(Jekyll::VERSION)
     end
 
+    context "with site.feed.updated = latest_post" do
+      let(:overrides) do
+        {
+          "feed" => {
+            "updated" => "latest_post",
+          },
+        }
+      end
+
+      it "outputs the last post time" do
+        expect(feed.updated.content).to eql(Time.parse("2016-04-25"))
+      end
+    end
+
+    it "outputs the current time" do
+      expect(feed.updated.content).to eql(Time.parse("2018-12-31"))
+    end
+
     it "includes the items" do
       expect(feed.items.count).to eql(10)
     end