no more errors when a blog has no created_at line, parse str as date instead of datetime

lib/helpers/blog.rb

@@ -30,10 +30,6 @@ module BlogHelper
     # find file last modification time
     filepath=@item[:content_filename]
     str=`git log -1 --format=%cd --date=short -- #{filepath}`
-    if str == ""
-        return @item[:created_at]
-    else
-        return str
-    end
+    return Date.parse(str)
   end
 end

lib/helpers/preprocess.rb

@@ -52,12 +52,10 @@ module PreprocessHelper
 
   def update_blog_attributes
     @items.find_all('/blog/**/*.md').each do |i|
-      raise "#{i.identifier} doesn't have 'created_at'" unless i[:created_at]
       i.update_attributes(
         # Tag all posts with article (for Blogging helper)
         kind: 'article',
         academic_year: i.identifier.to_s[/\d\d-\d\d/],
-        created_at: Date.parse(i[:created_at])
       )
     end
   end