diff --git a/packages/durabletask-js/src/worker/runtime-orchestration-context.ts b/packages/durabletask-js/src/worker/runtime-orchestration-context.ts
index 157e0366..64ae1083 100644
--- a/packages/durabletask-js/src/worker/runtime-orchestration-context.ts
+++ b/packages/durabletask-js/src/worker/runtime-orchestration-context.ts
@@ -296,13 +296,29 @@ export class RuntimeOrchestrationContext extends OrchestrationContext {
   createTimer(fireAt: number | Date): Task<any> {
     const id = this.nextSequenceNumber();
 
-    // If a number is passed, we use it as the number of seconds to wait
-    // we use instanceof Date as number is not a native Javascript type
-    if (!(fireAt instanceof Date)) {
-      fireAt = new Date(this._currentUtcDatetime.getTime() + fireAt * 1000);
+    let fireAtDate: Date;
+    if (typeof fireAt === "number") {
+      if (!Number.isFinite(fireAt)) {
+        throw new Error(
+          `createTimer requires a finite number (seconds) or a valid Date, but received ${String(fireAt)}`,
+        );
+      }
+      fireAtDate = new Date(this._currentUtcDatetime.getTime() + fireAt * 1000);
+    } else if (fireAt instanceof Date) {
+      fireAtDate = fireAt;
+    } else {
+      throw new Error(
+        `createTimer requires a finite number (seconds) or a valid Date, but received ${String(fireAt)}`,
+      );
+    }
+
+    if (Number.isNaN(fireAtDate.getTime())) {
+      throw new Error(
+        "createTimer received or produced an invalid Date (NaN timestamp)",
+      );
     }
 
-    const action = ph.newCreateTimerAction(id, fireAt);
+    const action = ph.newCreateTimerAction(id, fireAtDate);
     this._pendingActions[action.getId()] = action;
 
     const timerTask = new CompletableTask();
diff --git a/packages/durabletask-js/test/createtimer-validation.spec.ts b/packages/durabletask-js/test/createtimer-validation.spec.ts
new file mode 100644
index 00000000..d3c4166f
--- /dev/null
+++ b/packages/durabletask-js/test/createtimer-validation.spec.ts
@@ -0,0 +1,280 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+import { OrchestrationContext } from "../src/task/context/orchestration-context";
+import {
+  newExecutionStartedEvent,
+  newOrchestratorStartedEvent,
+} from "../src/utils/pb-helper.util";
+import { OrchestrationExecutor, OrchestrationExecutionResult } from "../src/worker/orchestration-executor";
+import * as pb from "../src/proto/orchestrator_service_pb";
+import { Registry } from "../src/worker/registry";
+import { TOrchestrator } from "../src/types/orchestrator.type";
+import { NoOpLogger } from "../src/types/logger.type";
+
+const testLogger = new NoOpLogger();
+const TEST_INSTANCE_ID = "test-createtimer-validation";
+
+/**
+ * Helper to extract the CompleteOrchestrationAction from an OrchestrationExecutionResult
+ */
+function getCompleteAction(
+  result: OrchestrationExecutionResult,
+): pb.CompleteOrchestrationAction | undefined {
+  const completeActions = result.actions.filter((a) => a.hasCompleteorchestration());
+  expect(completeActions.length).toEqual(1);
+  return completeActions[0].getCompleteorchestration() ?? undefined;
+}
+
+describe("createTimer input validation", () => {
+  it("should reject NaN as fireAt value", async () => {
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext) {
+      yield ctx.createTimer(NaN);
+    };
+
+    const registry = new Registry();
+    const name = registry.addOrchestrator(orchestrator);
+    const startTime = new Date("2025-01-01T00:00:00Z");
+    const newEvents = [
+      newOrchestratorStartedEvent(startTime),
+      newExecutionStartedEvent(name, TEST_INSTANCE_ID),
+    ];
+    const executor = new OrchestrationExecutor(registry, testLogger);
+    const result = await executor.execute(TEST_INSTANCE_ID, [], newEvents);
+
+    const completeAction = getCompleteAction(result);
+    expect(completeAction?.getOrchestrationstatus()).toEqual(
+      pb.OrchestrationStatus.ORCHESTRATION_STATUS_FAILED,
+    );
+    expect(completeAction?.getFailuredetails()?.getErrormessage()).toContain("createTimer");
+    expect(completeAction?.getFailuredetails()?.getErrormessage()).toContain("NaN");
+  });
+
+  it("should reject Infinity as fireAt value", async () => {
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext) {
+      yield ctx.createTimer(Infinity);
+    };
+
+    const registry = new Registry();
+    const name = registry.addOrchestrator(orchestrator);
+    const startTime = new Date("2025-01-01T00:00:00Z");
+    const newEvents = [
+      newOrchestratorStartedEvent(startTime),
+      newExecutionStartedEvent(name, TEST_INSTANCE_ID),
+    ];
+    const executor = new OrchestrationExecutor(registry, testLogger);
+    const result = await executor.execute(TEST_INSTANCE_ID, [], newEvents);
+
+    const completeAction = getCompleteAction(result);
+    expect(completeAction?.getOrchestrationstatus()).toEqual(
+      pb.OrchestrationStatus.ORCHESTRATION_STATUS_FAILED,
+    );
+    expect(completeAction?.getFailuredetails()?.getErrormessage()).toContain("createTimer");
+    expect(completeAction?.getFailuredetails()?.getErrormessage()).toContain("Infinity");
+  });
+
+  it("should reject negative Infinity as fireAt value", async () => {
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext) {
+      yield ctx.createTimer(-Infinity);
+    };
+
+    const registry = new Registry();
+    const name = registry.addOrchestrator(orchestrator);
+    const startTime = new Date("2025-01-01T00:00:00Z");
+    const newEvents = [
+      newOrchestratorStartedEvent(startTime),
+      newExecutionStartedEvent(name, TEST_INSTANCE_ID),
+    ];
+    const executor = new OrchestrationExecutor(registry, testLogger);
+    const result = await executor.execute(TEST_INSTANCE_ID, [], newEvents);
+
+    const completeAction = getCompleteAction(result);
+    expect(completeAction?.getOrchestrationstatus()).toEqual(
+      pb.OrchestrationStatus.ORCHESTRATION_STATUS_FAILED,
+    );
+    expect(completeAction?.getFailuredetails()?.getErrormessage()).toContain("createTimer");
+  });
+
+  it("should reject an invalid Date object", async () => {
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext) {
+      yield ctx.createTimer(new Date("not a date"));
+    };
+
+    const registry = new Registry();
+    const name = registry.addOrchestrator(orchestrator);
+    const startTime = new Date("2025-01-01T00:00:00Z");
+    const newEvents = [
+      newOrchestratorStartedEvent(startTime),
+      newExecutionStartedEvent(name, TEST_INSTANCE_ID),
+    ];
+    const executor = new OrchestrationExecutor(registry, testLogger);
+    const result = await executor.execute(TEST_INSTANCE_ID, [], newEvents);
+
+    const completeAction = getCompleteAction(result);
+    expect(completeAction?.getOrchestrationstatus()).toEqual(
+      pb.OrchestrationStatus.ORCHESTRATION_STATUS_FAILED,
+    );
+    expect(completeAction?.getFailuredetails()?.getErrormessage()).toContain("createTimer");
+    expect(completeAction?.getFailuredetails()?.getErrormessage()).toContain("invalid Date");
+  });
+
+  it("should reject values that are neither numbers nor Date objects", async () => {
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext) {
+      yield ctx.createTimer("not a timer" as unknown as number);
+    };
+
+    const registry = new Registry();
+    const name = registry.addOrchestrator(orchestrator);
+    const startTime = new Date("2025-01-01T00:00:00Z");
+    const newEvents = [
+      newOrchestratorStartedEvent(startTime),
+      newExecutionStartedEvent(name, TEST_INSTANCE_ID),
+    ];
+    const executor = new OrchestrationExecutor(registry, testLogger);
+    const result = await executor.execute(TEST_INSTANCE_ID, [], newEvents);
+
+    const completeAction = getCompleteAction(result);
+    expect(completeAction?.getOrchestrationstatus()).toEqual(
+      pb.OrchestrationStatus.ORCHESTRATION_STATUS_FAILED,
+    );
+    expect(completeAction?.getFailuredetails()?.getErrormessage()).toContain("createTimer");
+    expect(completeAction?.getFailuredetails()?.getErrormessage()).toContain("not a timer");
+  });
+
+  it("should reject a finite number that produces an invalid Date", async () => {
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext) {
+      yield ctx.createTimer(Number.MAX_VALUE);
+    };
+
+    const registry = new Registry();
+    const name = registry.addOrchestrator(orchestrator);
+    const startTime = new Date("2025-01-01T00:00:00Z");
+    const newEvents = [
+      newOrchestratorStartedEvent(startTime),
+      newExecutionStartedEvent(name, TEST_INSTANCE_ID),
+    ];
+    const executor = new OrchestrationExecutor(registry, testLogger);
+    const result = await executor.execute(TEST_INSTANCE_ID, [], newEvents);
+
+    const completeAction = getCompleteAction(result);
+    expect(completeAction?.getOrchestrationstatus()).toEqual(
+      pb.OrchestrationStatus.ORCHESTRATION_STATUS_FAILED,
+    );
+    expect(completeAction?.getFailuredetails()?.getErrormessage()).toContain("createTimer");
+    expect(completeAction?.getFailuredetails()?.getErrormessage()).toContain("invalid Date");
+  });
+
+  it("should accept a valid positive number (seconds)", async () => {
+    const delaySeconds = 30;
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext) {
+      yield ctx.createTimer(delaySeconds);
+    };
+
+    const registry = new Registry();
+    const name = registry.addOrchestrator(orchestrator);
+    const startTime = new Date("2025-01-01T00:00:00Z");
+    const expectedFireAt = new Date(startTime.getTime() + delaySeconds * 1000);
+    const newEvents = [
+      newOrchestratorStartedEvent(startTime),
+      newExecutionStartedEvent(name, TEST_INSTANCE_ID),
+    ];
+    const executor = new OrchestrationExecutor(registry, testLogger);
+    const result = await executor.execute(TEST_INSTANCE_ID, [], newEvents);
+
+    // Should have a createTimer action (not failed)
+    const timerActions = result.actions.filter((a) => a.hasCreatetimer());
+    expect(timerActions.length).toEqual(1);
+    const fireAt = timerActions[0].getCreatetimer()?.getFireat()?.toDate();
+    expect(fireAt?.getTime()).toEqual(expectedFireAt.getTime());
+  });
+
+  it("should accept zero as a valid delay (fires immediately)", async () => {
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext) {
+      yield ctx.createTimer(0);
+    };
+
+    const registry = new Registry();
+    const name = registry.addOrchestrator(orchestrator);
+    const startTime = new Date("2025-01-01T00:00:00Z");
+    const newEvents = [
+      newOrchestratorStartedEvent(startTime),
+      newExecutionStartedEvent(name, TEST_INSTANCE_ID),
+    ];
+    const executor = new OrchestrationExecutor(registry, testLogger);
+    const result = await executor.execute(TEST_INSTANCE_ID, [], newEvents);
+
+    // Zero is a valid delay — timer fires at the current orchestration time
+    const timerActions = result.actions.filter((a) => a.hasCreatetimer());
+    expect(timerActions.length).toEqual(1);
+    const fireAt = timerActions[0].getCreatetimer()?.getFireat()?.toDate();
+    expect(fireAt?.getTime()).toEqual(startTime.getTime());
+  });
+
+  it("should accept a valid Date object", async () => {
+    const futureDate = new Date("2025-06-15T12:00:00Z");
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext) {
+      yield ctx.createTimer(futureDate);
+    };
+
+    const registry = new Registry();
+    const name = registry.addOrchestrator(orchestrator);
+    const startTime = new Date("2025-01-01T00:00:00Z");
+    const newEvents = [
+      newOrchestratorStartedEvent(startTime),
+      newExecutionStartedEvent(name, TEST_INSTANCE_ID),
+    ];
+    const executor = new OrchestrationExecutor(registry, testLogger);
+    const result = await executor.execute(TEST_INSTANCE_ID, [], newEvents);
+
+    const timerActions = result.actions.filter((a) => a.hasCreatetimer());
+    expect(timerActions.length).toEqual(1);
+    const fireAt = timerActions[0].getCreatetimer()?.getFireat()?.toDate();
+    expect(fireAt?.getTime()).toEqual(futureDate.getTime());
+  });
+
+  it("should accept a negative number (timer fires in the past, which the sidecar handles)", async () => {
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext) {
+      yield ctx.createTimer(-5);
+    };
+
+    const registry = new Registry();
+    const name = registry.addOrchestrator(orchestrator);
+    const startTime = new Date("2025-01-01T00:00:00Z");
+    const expectedFireAt = new Date(startTime.getTime() - 5000);
+    const newEvents = [
+      newOrchestratorStartedEvent(startTime),
+      newExecutionStartedEvent(name, TEST_INSTANCE_ID),
+    ];
+    const executor = new OrchestrationExecutor(registry, testLogger);
+    const result = await executor.execute(TEST_INSTANCE_ID, [], newEvents);
+
+    // Negative delays create a timer in the past — sidecar fires it immediately.
+    // This is valid behavior for scenarios like conditional timer creation.
+    const timerActions = result.actions.filter((a) => a.hasCreatetimer());
+    expect(timerActions.length).toEqual(1);
+    const fireAt = timerActions[0].getCreatetimer()?.getFireat()?.toDate();
+    expect(fireAt?.getTime()).toEqual(expectedFireAt.getTime());
+  });
+
+  it("should accept fractional seconds", async () => {
+    const orchestrator: TOrchestrator = async function* (ctx: OrchestrationContext) {
+      yield ctx.createTimer(1.5);
+    };
+
+    const registry = new Registry();
+    const name = registry.addOrchestrator(orchestrator);
+    const startTime = new Date("2025-01-01T00:00:00Z");
+    const expectedFireAt = new Date(startTime.getTime() + 1500);
+    const newEvents = [
+      newOrchestratorStartedEvent(startTime),
+      newExecutionStartedEvent(name, TEST_INSTANCE_ID),
+    ];
+    const executor = new OrchestrationExecutor(registry, testLogger);
+    const result = await executor.execute(TEST_INSTANCE_ID, [], newEvents);
+
+    const timerActions = result.actions.filter((a) => a.hasCreatetimer());
+    expect(timerActions.length).toEqual(1);
+    const fireAt = timerActions[0].getCreatetimer()?.getFireat()?.toDate();
+    expect(fireAt?.getTime()).toEqual(expectedFireAt.getTime());
+  });
+});