add failing tests for #5963

it fails with ruby 2.4 onwards and passes up to ruby 2.3

test/helper.rb

@@ -149,7 +149,9 @@ def site_configuration(overrides = {})
     }))
     Configuration.from(full_overrides.merge({
       "source" => source_dir,
-    }))
+    })).tap do |obj|
+      Jekyll.set_timezone(obj["timezone"]) if obj["timezone"]
+    end
   end
 
   def clear_dest

test/source/_dates/date_without_time.md

@@ -0,0 +1,4 @@
+---
+date: 2015-10-01
+---
+Here is the content.

test/source/_dates/time_with_timezone.md

@@ -0,0 +1,4 @@
+---
+date: 2015-10-01 01:00:00 +0800
+---
+Here is the content.

test/source/_dates/time_without_timezone.md

@@ -0,0 +1,4 @@
+---
+date: 2015-10-01 01:00:00
+---
+Here is the content.

test/test_document.rb

@@ -16,6 +16,15 @@ def setup_encoded_document(filename)
     }).tap(&:read)
   end
 
+  def setup_document_with_dates(filename)
+    site = fixture_site("collections" => ["dates"], "timezone" => "UTC")
+    site.process
+    Document.new(site.in_source_dir(File.join("_dates", filename)), {
+      :site       => site,
+      :collection => site.collections["dates"],
+    }).tap(&:read)
+  end
+
   context "a document in a collection" do
     setup do
       @site = fixture_site({
@@ -558,4 +567,34 @@ def setup_encoded_document(filename)
       Jekyll::Renderer.new(@document.site, @document).render_document
     end
   end
+
+  context "a document with a date with timezone" do
+    setup do
+      @document = setup_document_with_dates "time_with_timezone.md"
+    end
+
+    should "have the expected date" do
+      assert_equal "2015/09/30", @document.data['date'].strftime("%Y/%m/%d")
+    end
+  end
+
+  context "a document with a date with time but without timezone" do
+    setup do
+      @document = setup_document_with_dates "time_without_timezone.md"
+    end
+
+    should "have the expected date" do
+      assert_equal "2015/10/01", @document.data['date'].strftime("%Y/%m/%d")
+    end
+  end
+
+  context "a document with a date without time" do
+    setup do
+      @document = setup_document_with_dates "date_without_time.md"
+    end
+
+    should "have the expected date" do
+      assert_equal "2015/10/01", @document.data['date'].strftime("%Y/%m/%d")
+    end
+  end
 end