date: fix date get timezone error while set utc time (#6495)

tests/by-util/test_date.rs

@@ -141,6 +141,16 @@ fn test_date_utc() {
     }
 }
 
+#[test]
+fn test_date_utc_issue_6495() {
+    new_ucmd!()
+        .arg("-u")
+        .arg("-d")
+        .arg("@0")
+        .succeeds()
+        .stdout_is("Thu Jan  1 00:00:00 1970\n");
+}
+
 #[test]
 fn test_date_format_y() {
     let scene = TestScenario::new(util_name!());