add datePublished to JSON-LD schema

lib/jekyll-seo-tag/drop.rb

@@ -102,6 +102,10 @@ module Jekyll
         end
       end
 
+      def date_published
+        @date_published ||= filters.date_to_xmlschema(page["date"]) if page["date"]
+      end
+
       def type
         @type ||= begin
           if page["seo"] && page["seo"]["type"]

lib/jekyll-seo-tag/json_ld.rb

@@ -4,18 +4,19 @@ module Jekyll
 
       # A hash of instance methods => key in resulting JSON-LD hash
       METHODS_KEYS = {
-        :json_context  => "@context",
-        :type          => "@type",
-        :name          => "name",
-        :page_title    => "headline",
-        :json_author   => "author",
-        :image_path    => "image",
-        :date_modified => "dateModified",
-        :description   => "description",
-        :publisher     => "publisher",
-        :main_entity   => "mainEntityOfPage",
-        :links         => "sameAs",
-        :canonical_url => "url",
+        :json_context   => "@context",
+        :type           => "@type",
+        :name           => "name",
+        :page_title     => "headline",
+        :json_author    => "author",
+        :image_path     => "image",
+        :date_published => "datePublished",
+        :date_modified  => "dateModified",
+        :description    => "description",
+        :publisher      => "publisher",
+        :main_entity    => "mainEntityOfPage",
+        :links          => "sameAs",
+        :canonical_url  => "url",
       }.freeze
 
       def json_ld

spec/jekyll_seo_tag/drop_spec.rb

@@ -246,6 +246,15 @@ RSpec.describe Jekyll::SeoTag::Drop do
     end
   end
 
+  context "date published" do
+    let(:config) { { "timezone" => "America/New_York" } }
+    let(:page_meta) { { "date" => "2017-01-01" } }
+
+    it "uses page.date" do
+      expect(subject.date_published).to eql("2017-01-01T00:00:00-05:00")
+    end
+  end
+
   context "date modified" do
     let(:config) { { "timezone" => "America/New_York" } }
 

spec/jekyll_seo_tag/json_ld_spec.rb

@@ -12,7 +12,7 @@ RSpec.describe Jekyll::SeoTag::JSONLD do
       "description" => "description",
       "seo"         => {
         "name"          => "seo name",
-        "date_modified" => "2017-01-01",
+        "date_modified" => "2017-01-02",
         "links"         => %w(a b),
       },
     }
@@ -62,9 +62,14 @@ RSpec.describe Jekyll::SeoTag::JSONLD do
     expect(subject["image"]).to eql("/image")
   end
 
+  it "returns the datePublished" do
+    expect(subject).to have_key("datePublished")
+    expect(subject["datePublished"]).to eql("2017-01-01T00:00:00-05:00")
+  end
+
   it "returns the dateModified" do
     expect(subject).to have_key("dateModified")
-    expect(subject["dateModified"]).to eql("2017-01-01T00:00:00-05:00")
+    expect(subject["dateModified"]).to eql("2017-01-02T00:00:00-05:00")
   end
 
   it "returns the description" do