[infra] Fix per-CL approval not setting the expires field.

Change-Id: Iaac00636aac6b755032fb19d2f17137c026ccf05
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/97623
Reviewed-by: William Hesse <whesse@google.com>
diff --git a/tools/approve_results.dart b/tools/approve_results.dart
index ac7e19f..67554a7 100755
--- a/tools/approve_results.dart
+++ b/tools/approve_results.dart
@@ -781,7 +781,8 @@
     exitCode = 1;
     return;
   }
-  final now = new DateTime.now().toUtc().toIso8601String();
+  final nowDate = new DateTime.now().toUtc();
+  final now = nowDate.toIso8601String();
 
   // Deep clones a decoded json object.
   dynamic deepClone(dynamic object) {
@@ -862,6 +863,8 @@
       preapproval["expected"] = test.expected;
       preapproval["preapprover"] = username;
       preapproval["preapproved_at"] = now;
+      preapproval["expires"] =
+          nowDate.add(const Duration(days: 30)).toIso8601String();
     }
   } else {
     // Import all the existing approval data for tests, removing tests that