diff --git a/Model/Resource.php b/Model/Resource.php
index 9975d7e..5af9d4c 100644
--- a/Model/Resource.php
+++ b/Model/Resource.php
@@ -11,7 +11,9 @@
 
 namespace FSi\Bundle\ResourceRepositoryBundle\Model;
 
+use DateTime;
 use DateTimeImmutable;
+use DateTimeInterface;
 
 class Resource implements ResourceValue
 {
@@ -82,7 +84,7 @@ public function getTextValue()
 
     public function setDateValue($dateValue)
     {
-        $this->dateValue = $dateValue;
+        $this->dateValue = $this->toDateTimeImmutable($dateValue);
     }
 
     public function getDateValue()
@@ -92,7 +94,7 @@ public function getDateValue()
 
     public function setDatetimeValue($datetimeValue)
     {
-        $this->datetimeValue = $datetimeValue;
+        $this->datetimeValue = $this->toDateTimeImmutable($datetimeValue);
     }
 
     public function getDatetimeValue()
@@ -102,7 +104,7 @@ public function getDatetimeValue()
 
     public function setTimeValue($timeValue)
     {
-        $this->timeValue = $timeValue;
+        $this->timeValue = $this->toDateTimeImmutable($timeValue);
     }
 
     public function getTimeValue()
@@ -139,4 +141,20 @@ public function getBoolValue()
     {
         return $this->boolValue;
     }
+
+    /**
+     * Symfony date/time/datetime forms do not allow for default DateTimeImmutable
+     * value until version 4.2, so the values need to be casted manually.
+     *
+     * @param DateTimeInterface|null $value
+     * @return DateTimeImmutable|null
+     */
+    private function toDateTimeImmutable(?DateTimeInterface $value): ?DateTimeImmutable
+    {
+        if (true === $value instanceof DateTime) {
+            $value = DateTimeImmutable::createFromMutable($value);
+        }
+
+        return $value;
+    }
 }
diff --git a/Model/ResourceValue.php b/Model/ResourceValue.php
index 72dc831..8eaa7c5 100644
--- a/Model/ResourceValue.php
+++ b/Model/ResourceValue.php
@@ -11,7 +11,7 @@
 
 namespace FSi\Bundle\ResourceRepositoryBundle\Model;
 
-Interface ResourceValue
+interface ResourceValue
 {
     /**
      * @param string $key
diff --git a/composer.json b/composer.json
index 81c5191..04afcd7 100644
--- a/composer.json
+++ b/composer.json
@@ -29,7 +29,7 @@
         "fsi/doctrine-extensions-bundle": "^1.0|^2.0",
         "fsi/form-extensions-bundle": "^2.0",
         "phpunit/phpunit": "^7.1",
-        "phpspec/phpspec": "^4.3",
+        "phpspec/phpspec": "^5.0",
         "phpspec/prophecy": "^1.7",
         "sebastian/exporter": "^3.1.0"
     },