diff --git a/src/Form/ItemType.php b/src/Form/ItemType.php
index b9501b5..2073deb 100644
--- a/src/Form/ItemType.php
+++ b/src/Form/ItemType.php
@@ -123,6 +123,8 @@ public function buildForm(FormBuilderInterface $builder, array $options) : void
             'choice_attr' => fn (Period $period, $key, $value) => ['data-year' => $period->getSortableYear()],
             'expanded' => false,
             'multiple' => false,
+            'required' => false,
+            'placeholder' => 'Unknown',
         ]);
         $builder->add('periodEnd', EntityType::class, [
             'label' => 'Latest creation',
@@ -131,6 +133,8 @@ public function buildForm(FormBuilderInterface $builder, array $options) : void
             'choice_attr' => fn (Period $period, $key, $value) => ['data-year' => $period->getSortableYear()],
             'expanded' => false,
             'multiple' => false,
+            'required' => false,
+            'placeholder' => 'Unknown',
         ]);
 
         $builder->add('provenance', Select2EntityType::class, [
diff --git a/templates/item/show.html.twig b/templates/item/show.html.twig
index 581c5b0..6a63272 100644
--- a/templates/item/show.html.twig
+++ b/templates/item/show.html.twig
@@ -128,11 +128,11 @@
                 {{ dl.make(item, 'dimensions', 'Dimensions') }}
                 {{ dl.make(item, 'location', 'Location') }}
                 {{ dl.make(item, 'civilization', 'Culture', 'civilization_show') }}
-                {% set display_period = [item.periodStart] %}
+                {% set display_period = [item.periodStart|default('Unknown')] %}
                 {% if not(item.periodEnd is same as(item.periodStart)) %}
-                    {% set display_period = display_period|merge([item.periodEnd]) %}
+                    {% set display_period = display_period|merge([item.periodEnd|default('Unknown')]) %}
                 {% endif %}
-                {{ dl.make({'display': display_period|join('-')|lower}, 'display', "Period") }}
+                {{ dl.make({'display': display_period|join(' - ')|lower}, 'display', "Period") }}
                 {{ dl.make(item, 'subjects', "Subjects", "subject_show") }}
             {% endset %}