diff --git a/lib/Property/ICalendar/Period.php b/lib/Property/ICalendar/Period.php
index 7632cb4d..c27da35c 100644
--- a/lib/Property/ICalendar/Period.php
+++ b/lib/Property/ICalendar/Period.php
@@ -72,6 +72,10 @@ function ($item) {
         parent::setJsonValue($value);
     }
 
+    function appendUtc($strDate) {
+        return strpos($strDate, 'Z') === false ? '' : 'Z';
+    }
+
     /**
      * Returns the value, in the format it should be encoded for json.
      *
@@ -85,19 +89,19 @@ public function getJsonValue(): array
         foreach ($this->getParts() as $item) {
             list($start, $end) = explode('/', $item, 2);
 
-            $start = DateTimeParser::parseDateTime($start);
-
             // This is a duration value.
-            if ('P' === $end[0]) {
+            $startDt = DateTimeParser::parseDateTime($start)->format('Y-m-d\\TH:i:s') . $this->appendUtc($start);
+
+            if ($end[0] === 'P') {
                 $return[] = [
-                    $start->format('Y-m-d\\TH:i:s'),
-                    $end,
+                    $startDt,
+                    $end
                 ];
             } else {
-                $end = DateTimeParser::parseDateTime($end);
+                $endDt = DateTimeParser::parseDateTime($end)->format('Y-m-d\\TH:i:s') . $this->appendUtc($end);
                 $return[] = [
-                    $start->format('Y-m-d\\TH:i:s'),
-                    $end->format('Y-m-d\\TH:i:s'),
+                    $startDt,
+                    $endDt
                 ];
             }
         }
diff --git a/tests/VObject/JCalTest.php b/tests/VObject/JCalTest.php
index fa347966..7624b8f8 100644
--- a/tests/VObject/JCalTest.php
+++ b/tests/VObject/JCalTest.php
@@ -97,7 +97,7 @@ public function testToJCal(): void
                             'sequence', new \stdClass(), 'integer', 5,
                         ],
                         [
-                            'freebusy', new \stdClass(), 'period',  ['2013-05-26T21:02:13', 'PT1H'], ['2013-06-26T12:00:00', '2013-06-26T13:00:00'],
+                            'freebusy', new \stdClass(), 'period',  ['2013-05-26T21:02:13Z', 'PT1H'], ['2013-06-26T12:00:00Z', '2013-06-26T13:00:00Z'],
                         ],
                         [
                             'url', new \stdClass(), 'uri', 'http://example.org/',
diff --git a/tests/VObject/Parser/JsonTest.php b/tests/VObject/Parser/JsonTest.php
index 392cc7c1..dacfb3b5 100644
--- a/tests/VObject/Parser/JsonTest.php
+++ b/tests/VObject/Parser/JsonTest.php
@@ -261,7 +261,7 @@ public function testRoundTripJCal(): void
                             'sequence', new \stdClass(), 'integer', 5,
                         ],
                         [
-                            'freebusy', new \stdClass(), 'period',  ['2013-05-26T21:02:13', 'PT1H'], ['2013-06-26T12:00:00', '2013-06-26T13:00:00'],
+                            'freebusy', new \stdClass(), 'period',  ['2013-05-26T21:02:13Z', 'PT1H'], ['2013-06-26T12:00:00Z', '2013-06-26T13:00:00Z'],
                         ],
                         [
                             'url', new \stdClass(), 'uri', 'http://example.org/',
diff --git a/tests/VObject/Parser/XmlTest.php b/tests/VObject/Parser/XmlTest.php
index 6491d087..f8b37f5c 100644
--- a/tests/VObject/Parser/XmlTest.php
+++ b/tests/VObject/Parser/XmlTest.php
@@ -684,7 +684,7 @@ public function testRFC6321Section3Part6Part9(): void
   <properties>
    <freebusy>
     <period>
-     <start>2011-05-17T12:00:00</start>
+     <start>2011-05-17T12:00:00Z</start>
      <duration>P1H</duration>
     </period>
    </freebusy>
@@ -693,7 +693,7 @@ public function testRFC6321Section3Part6Part9(): void
 </icalendar>
 XML,
             'BEGIN:VCALENDAR'."\n".
-            'FREEBUSY:20110517T120000/P1H'."\n".
+            'FREEBUSY:20110517T120000Z/P1H'."\n".
             'END:VCALENDAR'."\n"
         );
 
@@ -705,8 +705,8 @@ public function testRFC6321Section3Part6Part9(): void
   <properties>
    <freebusy>
     <period>
-     <start>2011-05-17T12:00:00</start>
-     <end>2012-05-17T12:00:00</end>
+     <start>2011-05-17T12:00:00Z</start>
+     <end>2012-05-17T12:00:00Z</end>
     </period>
    </freebusy>
   </properties>
@@ -714,7 +714,7 @@ public function testRFC6321Section3Part6Part9(): void
 </icalendar>
 XML,
             'BEGIN:VCALENDAR'."\n".
-            'FREEBUSY:20110517T120000/20120517T120000'."\n".
+            'FREEBUSY:20110517T120000Z/20120517T120000Z'."\n".
             'END:VCALENDAR'."\n"
         );
     }