diff --git a/.github/actions/time-to-first-review/index.js b/.github/actions/time-to-first-review/index.js
index 04e5b91..a10716c 100644
--- a/.github/actions/time-to-first-review/index.js
+++ b/.github/actions/time-to-first-review/index.js
@@ -1,36 +1,12 @@
 const core = require("@actions/core");
 
-const fetchPullRequests = require("./src/fetch-pull-requests");
-const getTimeToFirstReview = require("./src/get-time-to-first-review");
+const main = require("./src");
 
 try {
-  // GitHub repository information
   const repo = core.getInput("repo");
   const token = core.getInput("github_token");
-  const maxPRs = core.getInput("num_prs") || 200;
-
-  // Main function
-  const main = async () => {
-    const prs = await fetchPullRequests(repo, token, maxPRs);
-
-    let timesToFirstReview = await Promise.all(
-      prs.map((pr) => getTimeToFirstReview(pr, repo, token))
-    );
-
-    timesToFirstReview = timesToFirstReview.filter(
-      (time) => time !== undefined
-    );
-
-    if (timesToFirstReview.length > 0) {
-      const total = timesToFirstReview.reduce((sum, value) => sum + value, 0);
-      const average = total / timesToFirstReview.length;
-      console.log(`Average time to first review: ${average.toFixed(2)} hours`);
-    } else {
-      console.log(`No reviews found for the last ${maxPRs} PRs.`);
-    }
-  };
-
-  main();
+  const maxPRs = core.getInput("num_prs") || 100;
+  main(repo, token, maxPRs);
 } catch (error) {
   core.setFailed(error.message);
 }
diff --git a/.github/actions/time-to-first-review/src/__tests__/get-time-to-first-review.test.js b/.github/actions/time-to-first-review/src/__tests__/get-time-to-first-review.test.js
new file mode 100644
index 0000000..79ef8b7
--- /dev/null
+++ b/.github/actions/time-to-first-review/src/__tests__/get-time-to-first-review.test.js
@@ -0,0 +1,67 @@
+// Mock the fetchReviews function
+jest.mock("../fetch-reviews", () => jest.fn());
+
+const getTimeToFirstReview = require("../get-time-to-first-review");
+const fetchReviews = require("../fetch-reviews");
+
+const repo = "owner/repo";
+const gitHubToken = "my-token";
+
+describe.only("getTimeToFirstReview", () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+    jest.restoreAllMocks();
+  });
+
+  it("should return the time difference between the PR creation and the first review", async () => {
+    const pr = { number: 1, created_at: new Date("2024-01-01T00:00:00Z") };
+
+    const consoleLogSpy = jest.spyOn(console, "log").mockImplementation();
+
+    fetchReviews.mockResolvedValue([
+      { submitted_at: new Date("2024-01-01T01:00:00Z") },
+      { submitted_at: new Date("2024-01-01T02:00:00Z") },
+    ]);
+
+    const result = await getTimeToFirstReview(pr, repo, gitHubToken);
+
+    expect(result).toBe(1);
+    expect(consoleLogSpy).toHaveBeenCalledTimes(1);
+    expect(consoleLogSpy).toHaveBeenCalledWith(
+      "PR #1 - Time to first review: 1 hours"
+    );
+  });
+
+  it("should return 0 if there are no reviews", async () => {
+    const pr = { number: 1, created_at: new Date("2024-01-01T00:00:00Z") };
+
+    const consoleLogSpy = jest.spyOn(console, "log").mockImplementation();
+
+    fetchReviews.mockResolvedValue([]);
+
+    const result = await getTimeToFirstReview(pr, repo, gitHubToken);
+
+    expect(result).toBe(undefined);
+    expect(consoleLogSpy).toHaveBeenCalledTimes(1);
+    expect(consoleLogSpy).toHaveBeenCalledWith("PR #1 - No reviews found");
+  });
+
+  it("should return 0 if the first review is created before the PR", async () => {
+    const pr = { number: 1, created_at: new Date("2024-01-01T00:00:00Z") };
+
+    const consoleLogSpy = jest.spyOn(console, "log").mockImplementation();
+
+    fetchReviews.mockResolvedValue([
+      { submitted_at: new Date("2023-12-31T23:00:00Z") },
+      { submitted_at: new Date("2024-01-01T01:00:00Z") },
+    ]);
+
+    const result = await getTimeToFirstReview(pr, repo, gitHubToken);
+
+    expect(result).toBe(0);
+    expect(consoleLogSpy).toHaveBeenCalledTimes(1);
+    expect(consoleLogSpy).toHaveBeenCalledWith(
+      "PR #1 - Time to first review: 0 hours"
+    );
+  });
+});
diff --git a/.github/actions/time-to-first-review/src/__tests__/index.test.js b/.github/actions/time-to-first-review/src/__tests__/index.test.js
new file mode 100644
index 0000000..fb198d2
--- /dev/null
+++ b/.github/actions/time-to-first-review/src/__tests__/index.test.js
@@ -0,0 +1,42 @@
+jest.mock("../get-time-to-first-review");
+jest.mock("../fetch-pull-requests");
+
+const getTimeToFirstReview = require("../get-time-to-first-review");
+const fetchPullRequests = require("../fetch-pull-requests");
+
+const main = require("..");
+
+describe("main", () => {
+  const repo = "owner/repo";
+  const token = "token";
+  const maxPRs = 10;
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+    jest.restoreAllMocks();
+  });
+
+  it("should call getTimeToFirstReview and fetchPullRequests with the correct arguments", async () => {
+    fetchPullRequests.mockResolvedValue([
+      {
+        id: 1,
+      },
+      {
+        id: 2,
+      },
+    ]);
+
+    getTimeToFirstReview.mockResolvedValue(5);
+
+    const consoleLogSpy = jest.spyOn(console, "log").mockImplementation();
+
+    await main(repo, token, maxPRs);
+
+    expect(fetchPullRequests).toHaveBeenCalledWith(repo, token, maxPRs);
+    expect(getTimeToFirstReview).toHaveBeenCalledTimes(2);
+    expect(consoleLogSpy).toHaveBeenCalledTimes(1);
+    expect(consoleLogSpy).toHaveBeenCalledWith(
+      "Average time to first review: 5.00 hours"
+    );
+  });
+});
diff --git a/.github/actions/time-to-first-review/src/get-time-to-first-review.js b/.github/actions/time-to-first-review/src/get-time-to-first-review.js
index fa03dc4..3ded7db 100644
--- a/.github/actions/time-to-first-review/src/get-time-to-first-review.js
+++ b/.github/actions/time-to-first-review/src/get-time-to-first-review.js
@@ -8,18 +8,17 @@ module.exports = async (pr, repo, gitHubToken) => {
   // Find the first review date
   const firstReviewDate = reviews
     .map((review) => review.submitted_at)
-    .filter((date) => date) // filter out null or undefined
-    .sort()[0]; // Get the earliest review date
+    .filter((date) => date)
+    .sort()[0];
 
   if (!firstReviewDate) {
     console.log(`PR #${pr.number} - No reviews found`);
     return undefined;
   }
 
-  const timeToFirstReview = calculateTimeDifference(
-    prCreatedAt,
-    firstReviewDate
-  );
+  let timeToFirstReview = calculateTimeDifference(prCreatedAt, firstReviewDate);
+
+  timeToFirstReview = Math.max(timeToFirstReview, 0);
 
   console.log(
     `PR #${pr.number} - Time to first review: ${timeToFirstReview} hours`
diff --git a/.github/actions/time-to-first-review/src/index.js b/.github/actions/time-to-first-review/src/index.js
new file mode 100644
index 0000000..a35e672
--- /dev/null
+++ b/.github/actions/time-to-first-review/src/index.js
@@ -0,0 +1,20 @@
+const getTimeToFirstReview = require("./get-time-to-first-review");
+const fetchPullRequests = require("./fetch-pull-requests");
+
+module.exports = async (repo, token, maxPRs) => {
+  const prs = await fetchPullRequests(repo, token, maxPRs);
+
+  let timesToFirstReview = await Promise.all(
+    prs.map((pr) => getTimeToFirstReview(pr, repo, token))
+  );
+
+  timesToFirstReview = timesToFirstReview.filter((time) => time !== undefined);
+
+  if (timesToFirstReview.length > 0) {
+    const total = timesToFirstReview.reduce((sum, value) => sum + value, 0);
+    const average = total / timesToFirstReview.length;
+    console.log(`Average time to first review: ${average.toFixed(2)} hours`);
+  } else {
+    console.log(`No reviews found for the last ${maxPRs} PRs.`);
+  }
+};