diff --git a/src/DateTime/Period.php b/src/DateTime/Period.php
index edc9e0b..18bd227 100644
--- a/src/DateTime/Period.php
+++ b/src/DateTime/Period.php
@@ -7,8 +7,8 @@
 class Period
 {
     public function __construct(
-        public readonly \DateTimeImmutable $begin,
-        public readonly \DateTimeImmutable $end,
+        public readonly \DateTimeInterface $begin,
+        public readonly \DateTimeInterface $end,
     ) {
         if ($this->begin > $this->end) {
             throw new \LogicException();
diff --git a/src/DateTime/Phase.php b/src/DateTime/Phase.php
index c6d43ab..1db62ad 100644
--- a/src/DateTime/Phase.php
+++ b/src/DateTime/Phase.php
@@ -6,7 +6,7 @@
 
 enum Phase
 {
-    case before;
-    case after;
-    case onGoing;
+    case Before;
+    case After;
+    case OnGoing;
 }