From 70402ed855b2db6845b9c25d3149a04eb052567c Mon Sep 17 00:00:00 2001
From: Milind Chatrabhuji <milind.chatrabhuji@gmail.com>
Date: Mon, 26 Oct 2026 07:45:09 +0530
Subject: [PATCH] fix(schedules): normalize default UTC timezone before
 declarative schedule diff (#5066)

---
 .../createBackgroundWorker.server.test.ts     | 37 +++++++++++++++++++
 .../services/createBackgroundWorker.server.ts | 13 ++++---
 2 files changed, 45 insertions(+), 5 deletions(-)

diff --git a/apps/webapp/app/v3/services/createBackgroundWorker.server.test.ts b/apps/webapp/app/v3/services/createBackgroundWorker.server.test.ts
index 147721ecfef..91fdd1576a4 100644
--- a/apps/webapp/app/v3/services/createBackgroundWorker.server.test.ts
+++ b/apps/webapp/app/v3/services/createBackgroundWorker.server.test.ts
@@ -488,6 +488,43 @@ describe("syncDeclarativeSchedules registration", () => {
     }
   );
 
+  containerTest(
+    "preserves an existing Redis job when task schedule omits explicit UTC timezone",
+    async ({ prisma, redisOptions }) => {
+      const { project, prodEnv } = await seedProjectWithEnvs(prisma);
+      const schedule = await makeDeclarativeSchedule(prisma, project.id, [prodEnv.id]);
+      const updatedAt = schedule.updatedAt;
+      await seedScheduledTask(prisma, project.id, prodEnv.id);
+      const engine = createTestScheduleEngine(prisma, redisOptions);
+
+      try {
+        const jobId = scheduleJobId(schedule.instances[0].id);
+        await engine.registerNextTaskScheduleInstance({ instanceId: schedule.instances[0].id });
+        const before = await engine.getJob(jobId);
+
+        // Omit explicit timezone to verify default UTC normalization does not trigger false timing change
+        await syncDeclarativeSchedules(
+          declarativeTasks({ cron: "0 * * * *" }),
+          noWorker,
+          asEnv(prodEnv),
+          prisma,
+          engine
+        );
+
+        expect(before).toBeDefined();
+        expect(await engine.getJob(jobId)).toEqual(before);
+        expect(
+          await prisma.taskSchedule.findUniqueOrThrow({
+            where: { id: schedule.id },
+            select: { updatedAt: true },
+          })
+        ).toEqual({ updatedAt });
+      } finally {
+        await engine.quit();
+      }
+    }
+  );
+
   containerTest(
     "replaces the Redis job when declarative timing changes",
     async ({ prisma, redisOptions }) => {
diff --git a/apps/webapp/app/v3/services/createBackgroundWorker.server.ts b/apps/webapp/app/v3/services/createBackgroundWorker.server.ts
index bbdfee76e85..11ec228225f 100644
--- a/apps/webapp/app/v3/services/createBackgroundWorker.server.ts
+++ b/apps/webapp/app/v3/services/createBackgroundWorker.server.ts
@@ -1588,7 +1588,7 @@ export async function syncDeclarativeSchedules(
         environment.projectId,
         {
           cron: task.schedule.cron,
-          timezone: task.schedule.timezone,
+          timezone: task.schedule.timezone ?? "UTC",
           taskIdentifier: task.id,
           friendlyId: existingSchedule?.friendlyId,
           window: task.schedule.window,
@@ -1623,15 +1623,18 @@ export async function syncDeclarativeSchedules(
           windowPercentage: normalizedWindow.windowPercentage,
           defaultWindowDurationSeconds: existingSchedule.defaultWindowDurationSeconds,
         }).window;
+        const taskTimezone = task.schedule.timezone ?? "UTC";
+        const existingTimezone = existingSchedule.timezone ?? "UTC";
+        const timezoneChanged = existingTimezone !== taskTimezone;
         const timingChanged =
           existingSchedule.generatorExpression !== task.schedule.cron ||
-          existingSchedule.timezone !== task.schedule.timezone ||
+          timezoneChanged ||
           !scheduleWindowsEqual(previousWindow, nextWindow) ||
           // Clearing the minimum changes the effective range.
           existingSchedule.minimumWindowDurationSeconds !== minimumWindowDurationSeconds;
         const persistedValuesChanged =
           existingSchedule.generatorExpression !== task.schedule.cron ||
-          existingSchedule.timezone !== task.schedule.timezone ||
+          timezoneChanged ||
           existingSchedule.windowDurationSeconds !== normalizedWindow.windowDurationSeconds ||
           existingSchedule.windowPercentage !== normalizedWindow.windowPercentage ||
           existingSchedule.minimumWindowDurationSeconds !== minimumWindowDurationSeconds;
@@ -1643,7 +1646,7 @@ export async function syncDeclarativeSchedules(
               data: {
                 generatorExpression: task.schedule.cron,
                 generatorDescription: cronstrue.toString(task.schedule.cron),
-                timezone: task.schedule.timezone,
+                timezone: taskTimezone,
                 minimumWindowDurationSeconds,
                 ...normalizedWindow,
               },
@@ -1675,7 +1678,7 @@ export async function syncDeclarativeSchedules(
             taskIdentifier: task.id,
             generatorExpression: task.schedule.cron,
             generatorDescription: cronstrue.toString(task.schedule.cron),
-            timezone: task.schedule.timezone,
+            timezone: task.schedule.timezone ?? "UTC",
             type: "DECLARATIVE",
             minimumWindowDurationSeconds,
             ...normalizedWindow,