diff --git a/lib/__tests__/expected.yml b/lib/__tests__/expected.yml
index 86aa4561..f5003f6d 100644
--- a/lib/__tests__/expected.yml
+++ b/lib/__tests__/expected.yml
@@ -4288,7 +4288,7 @@ Resources:
     Properties:
       Code:
         S3Bucket: cdk-hnb659fds-assets-712950704752-us-east-1
-        S3Key: ff9251e1c9991e0788c269b3f4daac2ef4701a3e98de0bb0a1aedae2bb35a514.zip
+        S3Key: d81bf2d4c389ecd69472cf4b27599e1ffbe0c1543104f45d035834e863f64ca7.zip
       Role:
         Fn::GetAtt:
           - CodeCommitPipelineChangeControllerFunctionServiceRoleF02841DB
diff --git a/lib/change-control-lambda/time-window.ts b/lib/change-control-lambda/time-window.ts
index 16c7c03d..c035c133 100644
--- a/lib/change-control-lambda/time-window.ts
+++ b/lib/change-control-lambda/time-window.ts
@@ -39,6 +39,7 @@ type Events = { [uuid: string]: CalendarEvent };
  * @returns the events that represent the blocked time, or `undefined` if `now` is not "blocked".
  */
 export function shouldBlockPipeline(icalData: string | Buffer, now = new Date(), advanceMarginSec = 3600): CalendarEvent | undefined {
+  validateTz();
   const events: Events = ical.parseICS(icalData.toString('utf8'));
   const blocks = containingEventsWithMargin(events, now, advanceMarginSec);
   return blocks.length > 0 ? blocks[0] : undefined;
@@ -140,3 +141,9 @@ function overlaps(left: { start: Date; end: Date }, right: { start: Date; end: D
 function isBetween(date: Date, left: Date, right: Date): boolean {
   return date >= left && date <= right;
 }
+
+function validateTz() {
+  if (new Date().getTimezoneOffset() !== 0) {
+    throw new Error('Because of a bug in "node-ical", this module can only be used when the system time zone is set to UTC. Run this command again with "TZ=UTC"');
+  }
+}