diff --git a/lib/DateTimeAccessor.php b/lib/DateTimeAccessor.php
index aba9666..7769866 100644
--- a/lib/DateTimeAccessor.php
+++ b/lib/DateTimeAccessor.php
@@ -31,14 +31,14 @@
 class DateTimeAccessor
 {
 	/**
-	 * @var \DateTime
+	 * @var \DateTimeInterface
 	 */
 	private $datetime;
 
 	/**
-	 * @param \DateTime $datetime
+	 * @param \DateTimeInterface $datetime
 	 */
-	public function __construct(\DateTime $datetime)
+	public function __construct(\DateTimeInterface $datetime)
 	{
 		$this->datetime = $datetime;
 	}
diff --git a/lib/DateTimeFormatter.php b/lib/DateTimeFormatter.php
index 09269c2..efe4949 100644
--- a/lib/DateTimeFormatter.php
+++ b/lib/DateTimeFormatter.php
@@ -191,7 +191,7 @@ public function __invoke($datetime, $pattern_or_width_or_skeleton)
 	 * echo $datetime_formatter($datetime, ':Ehm');                     // Sat 10:23 PM
 	 * </pre>
 	 *
-	 * @param \DateTime|string|int $datetime The datetime to format.
+	 * @param \DateTimeInterface|string|int $datetime The datetime to format.
 	 * @param string $pattern_or_width_or_skeleton The datetime can be formatted using a pattern,
 	 * a width or a skeleton. The following width are defined: "full", "long", "medium" and "short".
 	 * To format the datetime using a so-called "skeleton", the skeleton identifier must be
diff --git a/lib/Territory.php b/lib/Territory.php
index 8f6c6b2..f16aa69 100644
--- a/lib/Territory.php
+++ b/lib/Territory.php
@@ -109,7 +109,7 @@ protected function lazy_get_currency()
 	/**
 	 * Return the currency used in the territory at a point in time.
 	 *
-	 * @param \DateTime|mixed $date
+	 * @param \DateTimeInterface|mixed $date
 	 *
 	 * @return Currency
 	 */
@@ -290,13 +290,13 @@ public function localize($locale_code)
 	}
 
 	/**
-	 * @param \DateTime|string $datetime
+	 * @param \DateTimeInterface|string $datetime
 	 *
-	 * @return \DateTime
+	 * @return \DateTimeInterface
 	 */
 	private function ensure_is_datetime($datetime)
 	{
-		return $datetime instanceof \DateTime
+		return $datetime instanceof \DateTimeInterface
 			? $datetime
 			: new \DateTime($datetime);
 	}