diff --git a/actions/setup/js/expiration_helpers.test.cjs b/actions/setup/js/expiration_helpers.test.cjs
new file mode 100644
index 0000000000..0b103bdedc
--- /dev/null
+++ b/actions/setup/js/expiration_helpers.test.cjs
@@ -0,0 +1,183 @@
+// @ts-check
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+
+describe("expiration_helpers", () => {
+  let mockCore;
+  let originalEnv;
+  let addExpirationComment;
+
+  beforeEach(async () => {
+    // Save original environment
+    originalEnv = { ...process.env };
+
+    // Reset mocks before each test
+    mockCore = {
+      info: vi.fn(),
+      messages: [],
+    };
+
+    // Capture logged messages
+    mockCore.info = vi.fn(msg => {
+      mockCore.messages.push({ level: "info", message: msg });
+    });
+
+    // Set globals
+    global.core = mockCore;
+
+    // Clear module cache and re-require to get fresh instance
+    vi.resetModules();
+    const module = await import("./expiration_helpers.cjs");
+    addExpirationComment = module.addExpirationComment;
+  });
+
+  afterEach(() => {
+    // Restore original environment
+    process.env = originalEnv;
+  });
+
+  describe("addExpirationComment", () => {
+    it("should add expiration comment when expires is set to valid hours", () => {
+      process.env.GH_AW_DISCUSSION_EXPIRES = "24";
+      const bodyLines = ["Line 1", "Line 2"];
+
+      addExpirationComment(bodyLines, "GH_AW_DISCUSSION_EXPIRES", "Discussion");
+
+      expect(bodyLines.length).toBe(3);
+      expect(bodyLines[2]).toContain("<!-- gh-aw-expires:");
+      expect(bodyLines[2]).toContain(" -->");
+      expect(mockCore.info).toHaveBeenCalledWith(expect.stringMatching(/Discussion will expire on .+ \(24 hours\)/));
+    });
+
+    it("should not add expiration comment when expires is not set", () => {
+      const bodyLines = ["Line 1", "Line 2"];
+
+      addExpirationComment(bodyLines, "GH_AW_DISCUSSION_EXPIRES", "Discussion");
+
+      expect(bodyLines.length).toBe(2);
+      expect(mockCore.info).not.toHaveBeenCalled();
+    });
+
+    it("should not add expiration comment when expires is invalid (NaN)", () => {
+      process.env.GH_AW_DISCUSSION_EXPIRES = "invalid";
+      const bodyLines = ["Line 1", "Line 2"];
+
+      addExpirationComment(bodyLines, "GH_AW_DISCUSSION_EXPIRES", "Discussion");
+
+      expect(bodyLines.length).toBe(2);
+      expect(mockCore.info).not.toHaveBeenCalled();
+    });
+
+    it("should not add expiration comment when expires is zero", () => {
+      process.env.GH_AW_DISCUSSION_EXPIRES = "0";
+      const bodyLines = ["Line 1", "Line 2"];
+
+      addExpirationComment(bodyLines, "GH_AW_DISCUSSION_EXPIRES", "Discussion");
+
+      expect(bodyLines.length).toBe(2);
+      expect(mockCore.info).not.toHaveBeenCalled();
+    });
+
+    it("should not add expiration comment when expires is negative", () => {
+      process.env.GH_AW_DISCUSSION_EXPIRES = "-5";
+      const bodyLines = ["Line 1", "Line 2"];
+
+      addExpirationComment(bodyLines, "GH_AW_DISCUSSION_EXPIRES", "Discussion");
+
+      expect(bodyLines.length).toBe(2);
+      expect(mockCore.info).not.toHaveBeenCalled();
+    });
+
+    it("should work with different environment variable names", () => {
+      process.env.GH_AW_ISSUE_EXPIRES = "48";
+      const bodyLines = ["Line 1"];
+
+      addExpirationComment(bodyLines, "GH_AW_ISSUE_EXPIRES", "Issue");
+
+      expect(bodyLines.length).toBe(2);
+      expect(bodyLines[1]).toContain("<!-- gh-aw-expires:");
+      expect(mockCore.info).toHaveBeenCalledWith(expect.stringMatching(/Issue will expire on .+ \(48 hours\)/));
+    });
+
+    it("should work with different entity types", () => {
+      process.env.GH_AW_PR_EXPIRES = "72";
+      const bodyLines = ["Line 1"];
+
+      addExpirationComment(bodyLines, "GH_AW_PR_EXPIRES", "Pull Request");
+
+      expect(bodyLines.length).toBe(2);
+      expect(bodyLines[1]).toContain("<!-- gh-aw-expires:");
+      expect(mockCore.info).toHaveBeenCalledWith(expect.stringMatching(/Pull Request will expire on .+ \(72 hours\)/));
+    });
+
+    it("should append to existing body lines", () => {
+      process.env.GH_AW_DISCUSSION_EXPIRES = "12";
+      const bodyLines = ["Existing line 1", "Existing line 2", "Existing line 3"];
+
+      addExpirationComment(bodyLines, "GH_AW_DISCUSSION_EXPIRES", "Discussion");
+
+      expect(bodyLines.length).toBe(4);
+      expect(bodyLines[0]).toBe("Existing line 1");
+      expect(bodyLines[1]).toBe("Existing line 2");
+      expect(bodyLines[2]).toBe("Existing line 3");
+      expect(bodyLines[3]).toContain("<!-- gh-aw-expires:");
+    });
+
+    it("should handle decimal hours by parsing to integer", () => {
+      process.env.GH_AW_DISCUSSION_EXPIRES = "24.5";
+      const bodyLines = ["Line 1"];
+
+      addExpirationComment(bodyLines, "GH_AW_DISCUSSION_EXPIRES", "Discussion");
+
+      expect(bodyLines.length).toBe(2);
+      expect(bodyLines[1]).toContain("<!-- gh-aw-expires:");
+      expect(mockCore.info).toHaveBeenCalledWith(expect.stringMatching(/Discussion will expire on .+ \(24 hours\)/));
+    });
+
+    it("should calculate correct expiration date", () => {
+      process.env.GH_AW_DISCUSSION_EXPIRES = "1";
+      const bodyLines = [];
+      const beforeTime = new Date();
+
+      addExpirationComment(bodyLines, "GH_AW_DISCUSSION_EXPIRES", "Discussion");
+
+      // Extract ISO date from the comment
+      const comment = bodyLines[0];
+      const match = comment.match(/<!-- gh-aw-expires: (.+?) -->/);
+      expect(match).not.toBeNull();
+
+      const expirationDate = new Date(match[1]);
+      const afterTime = new Date();
+      afterTime.setHours(afterTime.getHours() + 1);
+
+      // Expiration should be approximately 1 hour from now (allow 1 second variance for test execution)
+      const expectedMs = beforeTime.getTime() + 60 * 60 * 1000;
+      const actualMs = expirationDate.getTime();
+      expect(Math.abs(actualMs - expectedMs)).toBeLessThan(1000);
+    });
+
+    it("should handle empty body lines array", () => {
+      process.env.GH_AW_DISCUSSION_EXPIRES = "24";
+      const bodyLines = [];
+
+      addExpirationComment(bodyLines, "GH_AW_DISCUSSION_EXPIRES", "Discussion");
+
+      expect(bodyLines.length).toBe(1);
+      expect(bodyLines[0]).toContain("<!-- gh-aw-expires:");
+    });
+
+    it("should use ISO format for expiration date in comment", () => {
+      process.env.GH_AW_DISCUSSION_EXPIRES = "24";
+      const bodyLines = [];
+
+      addExpirationComment(bodyLines, "GH_AW_DISCUSSION_EXPIRES", "Discussion");
+
+      const comment = bodyLines[0];
+      const match = comment.match(/<!-- gh-aw-expires: (.+?) -->/);
+      expect(match).not.toBeNull();
+
+      // Verify it's a valid ISO 8601 date
+      const isoDate = match[1];
+      expect(isoDate).toMatch(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z$/);
+    });
+  });
+});