diff --git a/core/src/Core.php b/core/src/Core.php
index a05408126b..1d2061e943 100644
--- a/core/src/Core.php
+++ b/core/src/Core.php
@@ -5036,7 +5036,7 @@ public function toTimeStamp($str)
             $M = 0;
             $S = 0;
         }
-        $timeStamp = mktime($H, $M, $S, $m, $d, $Y);
+        $timeStamp = gmmktime($H, $M, $S, $m, $d, $Y);
         $timeStamp = (int) $timeStamp;
         return $timeStamp;
     }
diff --git a/core/tests/Unit/CoreTest.php b/core/tests/Unit/CoreTest.php
index 3f83f8fd18..653b57ad3c 100644
--- a/core/tests/Unit/CoreTest.php
+++ b/core/tests/Unit/CoreTest.php
@@ -73,6 +73,24 @@
     Mockery::close();
 });
 
+test('date conversion round trips without runtime timezone drift', function () {
+    $originalTimezone = date_default_timezone_get();
+
+    try {
+        date_default_timezone_set('Asia/Tokyo');
+        $this->parser->config['datetime_format'] = 'YYYY/mm/dd';
+
+        $date = '2026/09/01 10:00:00';
+        $timestamp = $this->parser->toTimeStamp($date);
+
+        expect($this->parser->toDateFormat($timestamp))->toBe($date)
+            ->and($this->parser->toDateFormat($this->parser->toTimeStamp($this->parser->toDateFormat($timestamp))))
+            ->toBe($date);
+    } finally {
+        date_default_timezone_set($originalTimezone);
+    }
+});
+
 describe('getTagsFromContent', function () {
 
     test('it extracts basic placeholders correctly', function () {