diff --git a/__tests__/api/timer.get.test.ts b/__tests__/api/timer.get.test.ts
index 7bd29a0..3989335 100644
--- a/__tests__/api/timer.get.test.ts
+++ b/__tests__/api/timer.get.test.ts
@@ -3,7 +3,7 @@ import { getServerSession } from 'next-auth';
 
 jest.mock('@/lib/prisma', () => ({
   prisma: {
-    user: { upsert: jest.fn() },
+    user: { upsert: jest.fn(), findUnique: jest.fn() },
     client: { findFirst: jest.fn() },
     timeEntry: { findFirst: jest.fn(), create: jest.fn(), update: jest.fn(), findUnique: jest.fn() },
   },
@@ -12,7 +12,7 @@ jest.mock('next-auth', () => ({ getServerSession: jest.fn() }));
 
 const { prisma: mockPrisma } = jest.requireMock('@/lib/prisma') as {
   prisma: {
-    user: { upsert: jest.Mock };
+    user: { upsert: jest.Mock; findUnique: jest.Mock };
     client: { findFirst: jest.Mock };
     timeEntry: { findFirst: jest.Mock; create: jest.Mock; update: jest.Mock; findUnique: jest.Mock };
   };
@@ -25,7 +25,7 @@ describe('GET /api/timer', () => {
 
   it('returns runningEntry when present', async () => {
     (getServerSession as jest.Mock).mockResolvedValue({ user: { email: 'test@example.com' } });
-    mockPrisma.user.upsert.mockResolvedValue({ id: 'user1', email: 'test@example.com' });
+    mockPrisma.user.findUnique.mockResolvedValue({ id: 'user1', email: 'test@example.com' });
     mockPrisma.timeEntry.findFirst.mockResolvedValue({ id: 'entry1', clientId: 'client1', startTime: new Date().toISOString(), status: 'RUNNING', client: { id: 'client1', name: 'C' } });
 
     const res = await GET();
@@ -38,7 +38,7 @@ describe('GET /api/timer', () => {
 
   it('returns null when no running entry', async () => {
     (getServerSession as jest.Mock).mockResolvedValue({ user: { email: 'test@example.com' } });
-    mockPrisma.user.upsert.mockResolvedValue({ id: 'user1', email: 'test@example.com' });
+    mockPrisma.user.findUnique.mockResolvedValue({ id: 'user1', email: 'test@example.com' });
     mockPrisma.timeEntry.findFirst.mockResolvedValue(null);
 
     const res = await GET();
diff --git a/__tests__/api/timer.post.test.ts b/__tests__/api/timer.post.test.ts
index 66d6cbf..ba34ee0 100644
--- a/__tests__/api/timer.post.test.ts
+++ b/__tests__/api/timer.post.test.ts
@@ -69,6 +69,8 @@ describe('POST /api/timer', () => {
     const res = await POST(req as Request);
     const json = await res.json();
 
-    expect(json.error).toBe('A timer is already running');
+    // With the simplified approach the POST returns the existing running entry
+    expect(json.runningEntry).toBeDefined();
+    expect(json.runningEntry.id).toBe('existing');
   });
 });
diff --git a/app/api/timer/route.ts b/app/api/timer/route.ts
index cf580f3..780cd77 100644
--- a/app/api/timer/route.ts
+++ b/app/api/timer/route.ts
@@ -41,13 +41,13 @@ export async function POST(req: Request) {
       return Response.json({ error: 'Client not found' }, { status: 404 });
     }
 
-    // Ensure there is no other running entry for this user
+    // If there is an existing running entry, return it (DB is the source of truth)
     const existingRunning = await prisma.timeEntry.findFirst({
       where: { userId: user.id, status: TimeEntryStatus.RUNNING },
     });
 
     if (existingRunning) {
-      return Response.json({ error: 'A timer is already running' }, { status: 409 });
+      return Response.json({ runningEntry: existingRunning });
     }
 
     // Create time entry with status RUNNING
@@ -77,24 +77,18 @@ export async function GET() {
   }
 
   try {
-    const user = await prisma.user.upsert({
-      where: { email: session.user.email },
-      update: {},
-      create: {
-        email: session.user.email,
-        name: session.user.name || '',
-      },
-    });
+    // Find the user read-only; do not create a user during GET so polling is read-only.
+    const user = await prisma.user.findUnique({ where: { email: session.user.email } });
+
+    if (!user) {
+      return Response.json({ runningEntry: null });
+    }
 
     const runningEntry = await prisma.timeEntry.findFirst({
-      where: {
-        userId: user.id,
-        status: 'RUNNING',
-      },
-      include: {
-        client: true,
-      },
+      where: { userId: user.id, status: TimeEntryStatus.RUNNING },
+      include: { client: true },
     });
+
     return Response.json({ runningEntry });
   } catch (error) {
     console.error('Error fetching in-progress time entries:', error);
diff --git a/app/timer/page.tsx b/app/timer/page.tsx
index cefcbd2..c67cead 100644
--- a/app/timer/page.tsx
+++ b/app/timer/page.tsx
@@ -1,7 +1,7 @@
-'use client';
+ 'use client';
 
-import { useEffect, useState } from 'react';
-import { useQuery } from '@tanstack/react-query';
+import { useMemo, useState, useEffect } from 'react';
+import { useQuery, useQueryClient } from '@tanstack/react-query';
 
 interface Client {
   id: string;
@@ -11,10 +11,38 @@ interface Client {
 
 export default function TimerPage() {
   const [selectedClientId, setSelectedClientId] = useState('');
-  const [isRunning, setIsRunning] = useState(false);
-  const [seconds, setSeconds] = useState(0);
-  const [startTime, setStartTime] = useState<Date | null>(null);
-  const [activeEntryId, setActiveEntryId] = useState<string | null>(null);
+  const queryClient = useQueryClient();
+
+  const { data: timerData } = useQuery({
+    queryKey: ['timerRunning'],
+    queryFn: async () => {
+      const res = await fetch('/api/timer');
+      if (!res.ok) throw new Error('Failed to fetch timer');
+      return res.json();
+    },
+    refetchInterval: (d: unknown) => {
+      const dd = d as Record<string, unknown> | null;
+      return dd && dd['runningEntry'] ? 1000 : false;
+    },
+  });
+
+  const runningEntry = timerData?.runningEntry ?? null;
+  const activeEntryId = runningEntry?.id ?? null;
+  const isRunning = !!runningEntry;
+  const [tick, setTick] = useState(() => Date.now());
+
+  // Update a lightweight tick every second while a timer is running to drive the display.
+  useEffect(() => {
+    if (!isRunning) return;
+    const id = setInterval(() => setTick(Date.now()), 1000);
+    return () => clearInterval(id);
+  }, [isRunning]);
+
+  const seconds = useMemo(() => {
+    if (!runningEntry?.startTime) return 0;
+    const diff = Math.floor((tick - new Date(runningEntry.startTime).getTime()) / 1000);
+    return Math.max(0, diff);
+  }, [runningEntry, tick]);
 
   const {
     data: clientsData,
@@ -31,44 +59,13 @@ export default function TimerPage() {
     },
   });
 
-  // On mount, check for any running entry and resume timer UI
+  // Polling via react-query handles fetching running entry every second.
+  // When a running entry exists, ensure selected client matches it.
   useEffect(() => {
-    let mounted = true;
-    (async () => {
-      try {
-        const res = await fetch('/api/timer');
-        if (!res.ok) return;
-        const data = await res.json();
-        const entry = data.runningEntry;
-        if (mounted && entry) {
-          setSelectedClientId(entry.clientId);
-          setActiveEntryId(entry.id);
-          const s = new Date(entry.startTime);
-          setStartTime(s);
-          const diff = Math.floor((Date.now() - s.getTime()) / 1000);
-          setSeconds(Math.max(0, diff));
-          setIsRunning(true);
-        }
-      } catch {
-        // ignore
-      }
-    })();
-    return () => {
-      mounted = false;
-    };
-  }, []);
-
-  useEffect(() => {
-    let interval: NodeJS.Timeout;
-
-    if (isRunning) {
-      interval = setInterval(() => {
-        setSeconds((s) => s + 1);
-      }, 1000);
+    if (runningEntry && selectedClientId !== runningEntry.clientId) {
+      setSelectedClientId(runningEntry.clientId);
     }
-
-    return () => clearInterval(interval);
-  }, [isRunning]);
+  }, [runningEntry, selectedClientId]);
 
   const handleStart = async () => {
     if (!selectedClientId) {
@@ -76,58 +73,50 @@ export default function TimerPage() {
       return;
     }
 
-    const now = new Date();
-    setStartTime(now);
-    setIsRunning(true);
-
-    // Create a TimeEntry in the database (without endTime)
-    const response = await fetch('/api/timer', {
-      method: 'POST',
-      headers: { 'Content-Type': 'application/json' },
-      body: JSON.stringify({
-        clientId: selectedClientId,
-        startTime: now.toISOString(),
-      }),
-    });
-
-    if (response.ok) {
-      const data = await response.json();
-      setActiveEntryId(data.timeEntry.id);
-    } else {
+    try {
+      const now = new Date().toISOString();
+      const res = await fetch('/api/timer', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ clientId: selectedClientId, startTime: now }),
+      });
+      if (!res.ok) throw new Error('Failed to start timer');
+      // invalidate query so the new running entry is fetched on next tick
+      queryClient.invalidateQueries({ queryKey: ['timerRunning'] });
+    } catch {
       alert('Error starting timer');
     }
   };
 
   const handleStop = async () => {
-    if (!startTime || !activeEntryId) return;
-
-    setIsRunning(false);
-
-    // Save to database
-    const endTime = new Date();
-    const response = await fetch(`/api/timer/${activeEntryId}`, {
-      method: 'PUT',
-      headers: { 'Content-Type': 'application/json' },
-      body: JSON.stringify({
-        endTime: endTime.toISOString(),
-      }),
-    });
-
-    if (response.ok) {
-      // Reset timer
-      setSeconds(0);
-      setStartTime(null);
-      setActiveEntryId(null);
+    if (!activeEntryId) return;
+
+    try {
+      const endTime = new Date().toISOString();
+      const res = await fetch(`/api/timer/${activeEntryId}`, {
+        method: 'PUT',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ endTime }),
+      });
+      if (!res.ok) throw new Error('Failed to stop');
+      queryClient.invalidateQueries({ queryKey: ['timerRunning'] });
       alert('Time entry saved!');
-    } else {
+    } catch {
       alert('Error saving time entry');
     }
   };
 
   const handleReset = () => {
-    setSeconds(0);
-    setIsRunning(false);
-    setStartTime(null);
+    // If there's an active server entry, stop it.
+    if (activeEntryId) {
+      handleStop();
+      return;
+    }
+
+    // Otherwise clear client selection and refresh timer query so UI shows 00:00:00
+    setSelectedClientId('');
+    setTick(Date.now());
+    queryClient.invalidateQueries({ queryKey: ['timerRunning'] });
   };
 
   const formatTime = (totalSeconds: number) => {
diff --git a/app/timer/useTimer.ts b/app/timer/useTimer.ts
new file mode 100644
index 0000000..76dbadd
--- /dev/null
+++ b/app/timer/useTimer.ts
@@ -0,0 +1,87 @@
+import { useEffect, useRef, useState } from 'react';
+
+type ResumeEntry = { id: string; startTime: string } | null;
+
+export function useTimer() {
+  const [isRunning, setIsRunning] = useState(false);
+  const [seconds, setSeconds] = useState(0);
+  const [startTime, setStartTime] = useState<Date | null>(null);
+  const [activeEntryId, setActiveEntryId] = useState<string | null>(null);
+  const intervalRef = useRef<NodeJS.Timeout | null>(null);
+
+  const MAX_RESUME_AGE_SECONDS = 24 * 60 * 60;
+
+  useEffect(() => {
+    if (intervalRef.current) {
+      clearInterval(intervalRef.current);
+      intervalRef.current = null;
+    }
+
+    if (isRunning) {
+      intervalRef.current = setInterval(() => {
+        if (!startTime) return;
+        const diff = Math.floor((Date.now() - startTime.getTime()) / 1000);
+        setSeconds(Math.max(0, diff));
+      }, 1000);
+    }
+
+    return () => {
+      if (intervalRef.current) {
+        clearInterval(intervalRef.current);
+        intervalRef.current = null;
+      }
+    };
+  }, [isRunning, startTime]);
+
+  const startLocal = (when: Date, entryId?: string | null) => {
+    // ensure any existing interval is cleared before starting
+    if (intervalRef.current) {
+      clearInterval(intervalRef.current);
+      intervalRef.current = null;
+    }
+    setStartTime(when);
+    setSeconds(0);
+    setActiveEntryId(entryId ?? null);
+    setIsRunning(true);
+  };
+
+  const stopLocal = () => {
+    // stop ticking immediately and clear interval
+    setIsRunning(false);
+    if (intervalRef.current) {
+      clearInterval(intervalRef.current);
+      intervalRef.current = null;
+    }
+  };
+
+  const resetLocal = () => {
+    setIsRunning(false);
+    setSeconds(0);
+    setStartTime(null);
+    setActiveEntryId(null);
+  };
+
+  const resumeFromEntry = (entry: ResumeEntry) => {
+    if (!entry) return;
+    const s = new Date(entry.startTime);
+    const diff = Math.floor((Date.now() - s.getTime()) / 1000);
+    if (diff > MAX_RESUME_AGE_SECONDS) return;
+    setStartTime(s);
+    setSeconds(Math.max(0, diff));
+    setActiveEntryId(entry.id);
+    setIsRunning(true);
+  };
+
+  return {
+    seconds,
+    isRunning,
+    startTime,
+    activeEntryId,
+    startLocal,
+    stopLocal,
+    resetLocal,
+    resumeFromEntry,
+  } as const;
+}
+
+export default useTimer;