diff --git a/app/models/metadata_document_date.rb b/app/models/metadata_document_date.rb
index 39e6873..7512cbf 100644
--- a/app/models/metadata_document_date.rb
+++ b/app/models/metadata_document_date.rb
@@ -8,16 +8,52 @@ class MetadataDocumentDate < MetadataTemplate
   end
 
   def value_from_document
+    return nil if post.new?
+
     document.date
   end
 
+  # Siempre es obligatorio
+  def required
+    true
+  end
+
+  def validate
+    super
+
+    errors << I18n.t('metadata.date.invalid_format') unless valid_format?
+
+    errors.empty?
+  end
+
   # El valor puede ser un Date, Time o una String en el formato
   # "yyyy-mm-dd"
+  #
+  # XXX: Date.iso8601 acepta fechas en el futuro lejano, como 20000,
+  # pero Jekyll las limita a cuatro cifras, así que vamos a mantener
+  # eso.
+  #
+  # @see {https://github.com/jekyll/jekyll/blob/master/lib/jekyll/document.rb#L15}
   def value
-    return (self[:value] = value_from_document || default_value) if self[:value].nil?
+    self[:value] =
+      case self[:value]
+      when String
+        begin
+          Date.iso8601(self[:value]).to_time
+        rescue Date::Error
+          value_from_document || default_value
+        end
+      else
+        self[:value] || value_from_document || default_value
+      end
+  end
 
-    self[:value] = Date.iso8601(self[:value]).to_time if self[:value].is_a? String
+  private
 
-    self[:value]
+  def valid_format?
+    return true if self[:value].is_a?(Time)
+
+    @valid_format_re ||= /\A\d{2,4}-\d{1,2}-\d{1,2}\z/
+    @valid_format_re =~ self[:value].to_s
   end
 end
diff --git a/app/models/post.rb b/app/models/post.rb
index d5f7709..31a269e 100644
--- a/app/models/post.rb
+++ b/app/models/post.rb
@@ -49,9 +49,7 @@ class Post
       public_send(attr)&.value = args[attr] if args.key?(attr)
     end
 
-    # XXX: No usamos Post#read porque a esta altura todavía no sabemos
-    # nada del Document
-    document.read! if File.exist? document.path
+    document.read! unless new?
   end
 
   def inspect