diff --git a/frontend/cntr/CountdownTimer/CountdownTimer.test.tsx b/frontend/cntr/CountdownTimer/CountdownTimer.test.tsx
new file mode 100644
index 0000000..dbc06b6
--- /dev/null
+++ b/frontend/cntr/CountdownTimer/CountdownTimer.test.tsx
@@ -0,0 +1,165 @@
+import React from "react";
+
+import {
+  render,
+  screen,
+  act,
+} from "@testing-library/react";
+
+import {
+  CountdownTimer,
+} from "./CountdownTimer";
+
+describe(
+  "CountdownTimer",
+  () => {
+    beforeEach(() => {
+      jest.useFakeTimers();
+
+      jest.setSystemTime(
+        new Date(
+          "2026-01-01T12:00:00Z",
+        ),
+      );
+    });
+
+    afterEach(() => {
+      jest.useRealTimers();
+    });
+
+    it(
+      "renders HH:MM:SS format",
+      () => {
+        render(
+          <CountdownTimer
+            endsAt={
+              new Date(
+                "2026-01-01T13:01:05Z",
+              )
+            }
+          />,
+        );
+
+        expect(
+          screen.getByText(
+            "01:01:05",
+          ),
+        ).toBeInTheDocument();
+      },
+    );
+
+    it(
+      "turns red when 5 minutes remain",
+      () => {
+        render(
+          <CountdownTimer
+            endsAt={
+              new Date(
+                "2026-01-01T12:05:00Z",
+              )
+            }
+          />,
+        );
+
+        expect(
+          screen.getByTestId(
+            "countdown-timer",
+          ),
+        ).toHaveClass(
+          "text-red-600",
+        );
+      },
+    );
+
+    it(
+      "calls onExpire exactly once",
+      () => {
+        const onExpire =
+          jest.fn();
+
+        render(
+          <CountdownTimer
+            endsAt={
+              new Date(
+                "2026-01-01T12:00:02Z",
+              )
+            }
+            onExpire={
+              onExpire
+            }
+          />,
+        );
+
+        act(() => {
+          jest.advanceTimersByTime(
+            5000,
+          );
+        });
+
+        expect(
+          onExpire,
+        ).toHaveBeenCalledTimes(
+          1,
+        );
+      },
+    );
+
+    it(
+      "updates every second",
+      () => {
+        render(
+          <CountdownTimer
+            endsAt={
+              new Date(
+                "2026-01-01T12:00:10Z",
+              )
+            }
+          />,
+        );
+
+        act(() => {
+          jest.advanceTimersByTime(
+            1000,
+          );
+        });
+
+        expect(
+          screen.getByText(
+            "00:00:09",
+          ),
+        ).toBeInTheDocument();
+      },
+    );
+
+    it(
+      "clears interval on unmount",
+      () => {
+        const clearSpy =
+          jest.spyOn(
+            window,
+            "clearInterval",
+          );
+
+        const {
+          unmount,
+        } = render(
+          <CountdownTimer
+            endsAt={
+              new Date(
+                "2026-01-01T12:00:10Z",
+              )
+            }
+          />,
+        );
+
+        unmount();
+
+        expect(
+          clearSpy,
+        ).toHaveBeenCalled();
+
+        clearSpy.mockRestore();
+      },
+    );
+  },
+);
\ No newline at end of file
diff --git a/frontend/cntr/CountdownTimer/CountdownTimer.tsx b/frontend/cntr/CountdownTimer/CountdownTimer.tsx
new file mode 100644
index 0000000..3dc4e09
--- /dev/null
+++ b/frontend/cntr/CountdownTimer/CountdownTimer.tsx
@@ -0,0 +1,151 @@
+import React, {
+  useEffect,
+  useMemo,
+  useRef,
+  useState,
+} from "react";
+
+export interface CountdownTimerProps {
+  endsAt: Date | string;
+  onExpire?: () => void;
+}
+
+const WARNING_THRESHOLD_SECONDS = 300;
+
+function getRemainingSeconds(
+  endsAt: Date | string,
+): number {
+  const endTime =
+    new Date(endsAt).getTime();
+
+  const now = Date.now();
+
+  return Math.max(
+    0,
+    Math.floor(
+      (endTime - now) / 1000,
+    ),
+  );
+}
+
+function formatTime(
+  totalSeconds: number,
+): string {
+  const hours = Math.floor(
+    totalSeconds / 3600,
+  );
+
+  const minutes = Math.floor(
+    (totalSeconds % 3600) / 60,
+  );
+
+  const seconds =
+    totalSeconds % 60;
+
+  return [
+    hours,
+    minutes,
+    seconds,
+  ]
+    .map((value) =>
+      String(value).padStart(
+        2,
+        "0",
+      ),
+    )
+    .join(":");
+}
+
+export const CountdownTimer =
+  ({
+    endsAt,
+    onExpire,
+  }: CountdownTimerProps) => {
+    const [
+      remainingSeconds,
+      setRemainingSeconds,
+    ] = useState(() =>
+      getRemainingSeconds(
+        endsAt,
+      ),
+    );
+
+    const hasExpiredRef =
+      useRef(false);
+
+    useEffect(() => {
+      setRemainingSeconds(
+        getRemainingSeconds(
+          endsAt,
+        ),
+      );
+
+      hasExpiredRef.current =
+        false;
+    }, [endsAt]);
+
+    useEffect(() => {
+      const tick = () => {
+        const remaining =
+          getRemainingSeconds(
+            endsAt,
+          );
+
+        setRemainingSeconds(
+          remaining,
+        );
+
+        if (
+          remaining === 0 &&
+          !hasExpiredRef.current
+        ) {
+          hasExpiredRef.current =
+            true;
+
+          onExpire?.();
+        }
+      };
+
+      tick();
+
+      const intervalId =
+        window.setInterval(
+          tick,
+          1000,
+        );
+
+      return () => {
+        window.clearInterval(
+          intervalId,
+        );
+      };
+    }, [endsAt, onExpire]);
+
+    const formattedTime =
+      useMemo(
+        () =>
+          formatTime(
+            remainingSeconds,
+          ),
+        [remainingSeconds],
+      );
+
+    const isWarning =
+      remainingSeconds <=
+      WARNING_THRESHOLD_SECONDS;
+
+    return (
+      <span
+        data-testid="countdown-timer"
+        className={
+          isWarning
+            ? "text-red-600 font-semibold"
+            : ""
+        }
+      >
+        {formattedTime}
+      </span>
+    );
+  };
+
+export default CountdownTimer;
\ No newline at end of file
diff --git a/frontend/cntr/CountdownTimer/index.ts b/frontend/cntr/CountdownTimer/index.ts
new file mode 100644
index 0000000..b0c8bc5
--- /dev/null
+++ b/frontend/cntr/CountdownTimer/index.ts
@@ -0,0 +1,4 @@
+export * from "./CountdownTimer";
+export {
+  default,
+} from "./CountdownTimer";
\ No newline at end of file