diff --git a/src/components/protected-layout.tsx b/src/components/protected-layout.tsx
index 96b2bc6..5ac716c 100644
--- a/src/components/protected-layout.tsx
+++ b/src/components/protected-layout.tsx
@@ -2,6 +2,7 @@ import { useEffect, type ReactNode } from "react";
 import { useNavigate } from "@tanstack/react-router";
 import { useAuth } from "@/lib/auth";
 import { AppShell } from "@/components/app-shell";
+import { TimerWidget } from "@/components/timer/timer-widget";
 import { Loader2 } from "lucide-react";
 
 export function ProtectedLayout({ children, adminOnly }: { children: ReactNode; adminOnly?: boolean }) {
@@ -25,5 +26,10 @@ export function ProtectedLayout({ children, adminOnly }: { children: ReactNode;
     );
   }
 
-  return <AppShell>{children}</AppShell>;
+  return (
+    <AppShell>
+      {children}
+      <TimerWidget />
+    </AppShell>
+  );
 }
diff --git a/src/components/timer/start-timer-button.tsx b/src/components/timer/start-timer-button.tsx
new file mode 100644
index 0000000..da1e588
--- /dev/null
+++ b/src/components/timer/start-timer-button.tsx
@@ -0,0 +1,57 @@
+import { useState } from "react";
+import { Button, type ButtonProps } from "@/components/ui/button";
+import { useTimer, type TimerTarget } from "@/lib/timer";
+import { Play, Timer as TimerIcon } from "lucide-react";
+import {
+  AlertDialog,
+  AlertDialogAction,
+  AlertDialogCancel,
+  AlertDialogContent,
+  AlertDialogDescription,
+  AlertDialogFooter,
+  AlertDialogHeader,
+  AlertDialogTitle,
+} from "@/components/ui/alert-dialog";
+import { formatHMS } from "@/lib/timer";
+
+interface Props extends Omit<ButtonProps, "onClick"> {
+  target: TimerTarget;
+  label?: string;
+}
+
+export function StartTimerButton({ target, label = "Start timer", ...rest }: Props) {
+  const { active, elapsedMs, start } = useTimer();
+  const [confirmOpen, setConfirmOpen] = useState(false);
+
+  const handleClick = () => {
+    if (active) setConfirmOpen(true);
+    else start(target);
+  };
+
+  return (
+    <>
+      <Button {...rest} onClick={handleClick}>
+        <Play className="h-4 w-4 mr-1.5" />
+        {label}
+      </Button>
+      <AlertDialog open={confirmOpen} onOpenChange={setConfirmOpen}>
+        <AlertDialogContent>
+          <AlertDialogHeader>
+            <AlertDialogTitle className="flex items-center gap-2">
+              <TimerIcon className="h-4 w-4" /> Replace running timer?
+            </AlertDialogTitle>
+            <AlertDialogDescription>
+              A timer is already running ({formatHMS(elapsedMs)}) on{" "}
+              <span className="font-medium">{active?.caseNumber ?? active?.clientName}</span>. Starting a new
+              one will discard it without saving.
+            </AlertDialogDescription>
+          </AlertDialogHeader>
+          <AlertDialogFooter>
+            <AlertDialogCancel>Keep current</AlertDialogCancel>
+            <AlertDialogAction onClick={() => start(target)}>Discard & start new</AlertDialogAction>
+          </AlertDialogFooter>
+        </AlertDialogContent>
+      </AlertDialog>
+    </>
+  );
+}
diff --git a/src/components/timer/timer-widget.tsx b/src/components/timer/timer-widget.tsx
new file mode 100644
index 0000000..9ec855c
--- /dev/null
+++ b/src/components/timer/timer-widget.tsx
@@ -0,0 +1,173 @@
+import { useState } from "react";
+import { useTimer, formatHMS } from "@/lib/timer";
+import { Button } from "@/components/ui/button";
+import { Input } from "@/components/ui/input";
+import { Label } from "@/components/ui/label";
+import { Textarea } from "@/components/ui/textarea";
+import { Checkbox } from "@/components/ui/checkbox";
+import { Dialog, DialogContent, DialogFooter, DialogHeader, DialogTitle, DialogDescription } from "@/components/ui/dialog";
+import { Pause, Play, Square, Timer as TimerIcon, Loader2 } from "lucide-react";
+import { useAuth } from "@/lib/auth";
+import { supabase } from "@/integrations/supabase/client";
+import { toast } from "sonner";
+import { Link } from "@tanstack/react-router";
+
+export function TimerWidget() {
+  const { active, elapsedMs, isRunning, pause, resume, setDescription, stopAndConsume, reset } = useTimer();
+  const { user, session } = useAuth();
+  const [stopOpen, setStopOpen] = useState(false);
+  const [saving, setSaving] = useState(false);
+  const [form, setForm] = useState({ hours: "", rate: "", description: "", billable: true, work_date: "" });
+
+  if (!session || !active) return null;
+
+  const openStop = () => {
+    if (isRunning) pause();
+    const snapshot = {
+      hours: (elapsedMs / 3600000).toFixed(2),
+      rate: active.defaultHourlyRate?.toString() ?? "",
+      description: active.description,
+      billable: true,
+      work_date: new Date().toISOString().slice(0, 10),
+    };
+    setForm(snapshot);
+    setStopOpen(true);
+  };
+
+  const cancelStop = () => setStopOpen(false);
+
+  const discard = () => {
+    if (!confirm("Discard this timer without saving?")) return;
+    reset();
+    setStopOpen(false);
+  };
+
+  const saveEntry = async (e: React.FormEvent) => {
+    e.preventDefault();
+    if (!user?.id) return;
+    if (!active.caseId) {
+      toast.error("Attach the timer to a case before saving a time entry.");
+      return;
+    }
+    const hours = parseFloat(form.hours);
+    const rate = parseFloat(form.rate || "0");
+    if (!hours || hours <= 0) return toast.error("Hours must be greater than 0");
+    if (!form.description.trim()) return toast.error("Description required");
+    setSaving(true);
+    const { error } = await supabase.from("time_entries").insert({
+      case_id: active.caseId,
+      user_id: user.id,
+      work_date: form.work_date,
+      hours,
+      hourly_rate: rate,
+      description: form.description.trim(),
+      billable: form.billable,
+    });
+    setSaving(false);
+    if (error) return toast.error(error.message);
+    toast.success("Time entry saved");
+    stopAndConsume();
+    setStopOpen(false);
+  };
+
+  const subtitle = active.caseId
+    ? `${active.caseNumber ?? ""} · ${active.caseTitle ?? ""}`
+    : `${active.clientName} (no case)`;
+
+  return (
+    <>
+      <div className="fixed bottom-4 right-4 z-50 w-[320px] rounded-lg border bg-card text-card-foreground shadow-lg">
+        <div className="flex items-center gap-2 px-3 py-2 border-b">
+          <TimerIcon className="h-4 w-4 text-primary" />
+          <span className="text-xs uppercase tracking-wider text-muted-foreground">Active timer</span>
+          <span className={`ml-auto inline-block h-2 w-2 rounded-full ${isRunning ? "bg-emerald-500 animate-pulse" : "bg-muted-foreground/40"}`} />
+        </div>
+        <div className="px-3 pt-3 pb-2">
+          <div className="font-mono text-3xl tabular-nums tracking-tight">{formatHMS(elapsedMs)}</div>
+          <div className="mt-1 text-xs text-muted-foreground truncate">
+            {active.caseId ? (
+              <Link to="/cases/$caseId" params={{ caseId: active.caseId }} className="hover:text-primary underline-offset-4 hover:underline">
+                {subtitle}
+              </Link>
+            ) : (
+              <Link to="/clients/$clientId" params={{ clientId: active.clientId }} className="hover:text-primary underline-offset-4 hover:underline">
+                {subtitle}
+              </Link>
+            )}
+          </div>
+          {!active.caseId && (
+            <div className="mt-1 text-[10px] text-destructive">Attach to a case to save as a time entry.</div>
+          )}
+        </div>
+        <div className="flex gap-2 px-3 pb-3">
+          {isRunning ? (
+            <Button size="sm" variant="secondary" className="flex-1" onClick={pause}>
+              <Pause className="h-4 w-4 mr-1.5" /> Pause
+            </Button>
+          ) : (
+            <Button size="sm" className="flex-1" onClick={resume}>
+              <Play className="h-4 w-4 mr-1.5" /> Resume
+            </Button>
+          )}
+          <Button size="sm" variant="outline" className="flex-1" onClick={openStop}>
+            <Square className="h-4 w-4 mr-1.5" /> Stop
+          </Button>
+        </div>
+      </div>
+
+      <Dialog open={stopOpen} onOpenChange={setStopOpen}>
+        <DialogContent className="sm:max-w-md">
+          <DialogHeader>
+            <DialogTitle>Save time entry</DialogTitle>
+            <DialogDescription>{subtitle}</DialogDescription>
+          </DialogHeader>
+          <form onSubmit={saveEntry} className="space-y-3">
+            <div className="grid grid-cols-2 gap-3">
+              <div className="space-y-1.5">
+                <Label className="text-xs">Date</Label>
+                <Input type="date" value={form.work_date} onChange={(e) => setForm({ ...form, work_date: e.target.value })} required />
+              </div>
+              <div className="space-y-1.5">
+                <Label className="text-xs">Hours</Label>
+                <Input type="number" step="0.01" min="0.01" value={form.hours} onChange={(e) => setForm({ ...form, hours: e.target.value })} required />
+              </div>
+            </div>
+            <div className="space-y-1.5">
+              <Label className="text-xs">Hourly rate ($)</Label>
+              <Input type="number" step="0.01" min="0" value={form.rate} onChange={(e) => setForm({ ...form, rate: e.target.value })} required />
+            </div>
+            <div className="space-y-1.5">
+              <Label className="text-xs">Description</Label>
+              <Textarea
+                rows={3}
+                value={form.description}
+                onChange={(e) => {
+                  setForm({ ...form, description: e.target.value });
+                  setDescription(e.target.value);
+                }}
+                required
+                maxLength={1000}
+              />
+            </div>
+            <label className="flex items-center gap-2 text-sm">
+              <Checkbox checked={form.billable} onCheckedChange={(c) => setForm({ ...form, billable: !!c })} />
+              Billable
+            </label>
+            <DialogFooter className="gap-2 sm:justify-between">
+              <Button type="button" variant="ghost" className="text-destructive" onClick={discard}>
+                Discard
+              </Button>
+              <div className="flex gap-2">
+                <Button type="button" variant="outline" onClick={cancelStop}>Keep timing</Button>
+                <Button type="submit" disabled={saving || !active.caseId}>
+                  {saving && <Loader2 className="h-4 w-4 mr-2 animate-spin" />}
+                  Save entry
+                </Button>
+              </div>
+            </DialogFooter>
+          </form>
+        </DialogContent>
+      </Dialog>
+    </>
+  );
+}
diff --git a/src/lib/timer.tsx b/src/lib/timer.tsx
new file mode 100644
index 0000000..2506bfa
--- /dev/null
+++ b/src/lib/timer.tsx
@@ -0,0 +1,189 @@
+import { createContext, useContext, useEffect, useRef, useState, type ReactNode } from "react";
+
+const STORAGE_KEY = "counsel.timer.v1";
+
+export interface TimerTarget {
+  clientId: string;
+  clientName: string;
+  caseId?: string | null;
+  caseNumber?: string | null;
+  caseTitle?: string | null;
+  defaultHourlyRate?: number | null;
+}
+
+interface PersistedTimer extends TimerTarget {
+  // Total accumulated milliseconds while running, excluding current segment
+  accumulatedMs: number;
+  // Timestamp (ms) when the current running segment started, or null if paused
+  runningSince: number | null;
+  description: string;
+  startedAt: string; // ISO when first started
+}
+
+interface TimerState {
+  active: PersistedTimer | null;
+  elapsedMs: number;
+  isRunning: boolean;
+  start: (target: TimerTarget, description?: string) => void;
+  pause: () => void;
+  resume: () => void;
+  reset: () => void;
+  setDescription: (d: string) => void;
+  // Returns elapsed hours rounded to 2 decimals; clears timer
+  stopAndConsume: () => { hours: number; description: string; target: TimerTarget; startedAt: string } | null;
+}
+
+const TimerContext = createContext<TimerState | undefined>(undefined);
+
+function read(): PersistedTimer | null {
+  if (typeof window === "undefined") return null;
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY);
+    if (!raw) return null;
+    return JSON.parse(raw) as PersistedTimer;
+  } catch {
+    return null;
+  }
+}
+
+function write(t: PersistedTimer | null) {
+  if (typeof window === "undefined") return;
+  if (!t) localStorage.removeItem(STORAGE_KEY);
+  else localStorage.setItem(STORAGE_KEY, JSON.stringify(t));
+}
+
+function computeElapsed(t: PersistedTimer | null): number {
+  if (!t) return 0;
+  return t.accumulatedMs + (t.runningSince ? Date.now() - t.runningSince : 0);
+}
+
+export function TimerProvider({ children }: { children: ReactNode }) {
+  const [active, setActive] = useState<PersistedTimer | null>(null);
+  const [elapsedMs, setElapsedMs] = useState(0);
+  const intervalRef = useRef<number | null>(null);
+
+  // Hydrate from localStorage on mount
+  useEffect(() => {
+    const t = read();
+    if (t) {
+      setActive(t);
+      setElapsedMs(computeElapsed(t));
+    }
+  }, []);
+
+  // Tick every second when running
+  useEffect(() => {
+    if (intervalRef.current) {
+      clearInterval(intervalRef.current);
+      intervalRef.current = null;
+    }
+    if (active?.runningSince) {
+      intervalRef.current = window.setInterval(() => {
+        setElapsedMs(computeElapsed(active));
+      }, 1000);
+    }
+    return () => {
+      if (intervalRef.current) clearInterval(intervalRef.current);
+    };
+  }, [active]);
+
+  // Cross-tab sync
+  useEffect(() => {
+    const onStorage = (e: StorageEvent) => {
+      if (e.key !== STORAGE_KEY) return;
+      const t = read();
+      setActive(t);
+      setElapsedMs(computeElapsed(t));
+    };
+    window.addEventListener("storage", onStorage);
+    return () => window.removeEventListener("storage", onStorage);
+  }, []);
+
+  const persist = (t: PersistedTimer | null) => {
+    write(t);
+    setActive(t);
+    setElapsedMs(computeElapsed(t));
+  };
+
+  const start: TimerState["start"] = (target, description = "") => {
+    const t: PersistedTimer = {
+      ...target,
+      accumulatedMs: 0,
+      runningSince: Date.now(),
+      description,
+      startedAt: new Date().toISOString(),
+    };
+    persist(t);
+  };
+
+  const pause = () => {
+    if (!active?.runningSince) return;
+    const accumulatedMs = active.accumulatedMs + (Date.now() - active.runningSince);
+    persist({ ...active, accumulatedMs, runningSince: null });
+  };
+
+  const resume = () => {
+    if (!active || active.runningSince) return;
+    persist({ ...active, runningSince: Date.now() });
+  };
+
+  const reset = () => persist(null);
+
+  const setDescription = (description: string) => {
+    if (!active) return;
+    persist({ ...active, description });
+  };
+
+  const stopAndConsume: TimerState["stopAndConsume"] = () => {
+    if (!active) return null;
+    const ms = computeElapsed(active);
+    const hours = Math.max(0.01, Math.round((ms / 3600000) * 100) / 100);
+    const result = {
+      hours,
+      description: active.description,
+      target: {
+        clientId: active.clientId,
+        clientName: active.clientName,
+        caseId: active.caseId,
+        caseNumber: active.caseNumber,
+        caseTitle: active.caseTitle,
+        defaultHourlyRate: active.defaultHourlyRate,
+      },
+      startedAt: active.startedAt,
+    };
+    persist(null);
+    return result;
+  };
+
+  return (
+    <TimerContext.Provider
+      value={{
+        active,
+        elapsedMs,
+        isRunning: !!active?.runningSince,
+        start,
+        pause,
+        resume,
+        reset,
+        setDescription,
+        stopAndConsume,
+      }}
+    >
+      {children}
+    </TimerContext.Provider>
+  );
+}
+
+export function useTimer() {
+  const ctx = useContext(TimerContext);
+  if (!ctx) throw new Error("useTimer must be used within TimerProvider");
+  return ctx;
+}
+
+export function formatHMS(ms: number): string {
+  const total = Math.floor(ms / 1000);
+  const h = Math.floor(total / 3600);
+  const m = Math.floor((total % 3600) / 60);
+  const s = total % 60;
+  return `${String(h).padStart(2, "0")}:${String(m).padStart(2, "0")}:${String(s).padStart(2, "0")}`;
+}
diff --git a/src/routes/__root.tsx b/src/routes/__root.tsx
index de705b2..be672d6 100644
--- a/src/routes/__root.tsx
+++ b/src/routes/__root.tsx
@@ -1,5 +1,6 @@
 import { Outlet, Link, createRootRoute, HeadContent, Scripts } from "@tanstack/react-router";
 import { AuthProvider } from "@/lib/auth";
+import { TimerProvider } from "@/lib/timer";
 
 import appCss from "../styles.css?url";
 
@@ -70,7 +71,9 @@ function RootShell({ children }: { children: React.ReactNode }) {
 function RootComponent() {
   return (
     <AuthProvider>
-      <Outlet />
+      <TimerProvider>
+        <Outlet />
+      </TimerProvider>
     </AuthProvider>
   );
 }
diff --git a/src/routes/cases.$caseId.tsx b/src/routes/cases.$caseId.tsx
index 27741b1..19aecdb 100644
--- a/src/routes/cases.$caseId.tsx
+++ b/src/routes/cases.$caseId.tsx
@@ -18,6 +18,7 @@ import { CaseInvoicesTab } from "@/components/cases/invoices-tab";
 import { CaseLitigationTab } from "@/components/cases/litigation-tab";
 import { toast } from "sonner";
 import { useAuth } from "@/lib/auth";
+import { StartTimerButton } from "@/components/timer/start-timer-button";
 
 export const Route = createFileRoute("/cases/$caseId")({
   component: () => (
@@ -105,27 +106,41 @@ function CaseDetail() {
             {" · "}{data.practice_area || "—"}{" · "}opened {formatDate(data.opened_at)}
           </p>
         </div>
-        {canManage && (
-          <div className="flex flex-wrap gap-2">
-            <Select value={data.assigned_attorney_id ?? ""} onValueChange={updateAssignee}>
-              <SelectTrigger className="w-[200px]"><SelectValue placeholder="Assign attorney" /></SelectTrigger>
-              <SelectContent>
-                {users.map((u) => <SelectItem key={u.id} value={u.id}>{u.full_name || u.email}</SelectItem>)}
-              </SelectContent>
-            </Select>
-            <Select value={data.status} onValueChange={updateStatus}>
-              <SelectTrigger className="w-[160px]"><SelectValue /></SelectTrigger>
-              <SelectContent>
-                <SelectItem value="intake">Intake</SelectItem>
-                <SelectItem value="active">Active</SelectItem>
-                <SelectItem value="on_hold">On hold</SelectItem>
-                <SelectItem value="closed_won">Closed — won</SelectItem>
-                <SelectItem value="closed_lost">Closed — lost</SelectItem>
-                <SelectItem value="closed">Closed</SelectItem>
-              </SelectContent>
-            </Select>
-          </div>
-        )}
+        <div className="flex flex-wrap gap-2">
+          <StartTimerButton
+            size="sm"
+            variant="outline"
+            target={{
+              clientId: data.client.id,
+              clientName: data.client.name,
+              caseId: data.id,
+              caseNumber: data.case_number,
+              caseTitle: data.title,
+              defaultHourlyRate: data.default_hourly_rate,
+            }}
+          />
+          {canManage && (
+            <>
+              <Select value={data.assigned_attorney_id ?? ""} onValueChange={updateAssignee}>
+                <SelectTrigger className="w-[200px]"><SelectValue placeholder="Assign attorney" /></SelectTrigger>
+                <SelectContent>
+                  {users.map((u) => <SelectItem key={u.id} value={u.id}>{u.full_name || u.email}</SelectItem>)}
+                </SelectContent>
+              </Select>
+              <Select value={data.status} onValueChange={updateStatus}>
+                <SelectTrigger className="w-[160px]"><SelectValue /></SelectTrigger>
+                <SelectContent>
+                  <SelectItem value="intake">Intake</SelectItem>
+                  <SelectItem value="active">Active</SelectItem>
+                  <SelectItem value="on_hold">On hold</SelectItem>
+                  <SelectItem value="closed_won">Closed — won</SelectItem>
+                  <SelectItem value="closed_lost">Closed — lost</SelectItem>
+                  <SelectItem value="closed">Closed</SelectItem>
+                </SelectContent>
+              </Select>
+            </>
+          )}
+        </div>
       </div>
 
       {data.description && (
diff --git a/src/routes/clients.$clientId.tsx b/src/routes/clients.$clientId.tsx
index 72ec9d9..9102758 100644
--- a/src/routes/clients.$clientId.tsx
+++ b/src/routes/clients.$clientId.tsx
@@ -10,6 +10,7 @@ import { ClientFormDialog } from "@/components/clients/client-form-dialog";
 import { useAuth } from "@/lib/auth";
 import { ArrowLeft, Edit, Plus, Building2, User, Briefcase as Building, MapPin, Mail, Phone, Users } from "lucide-react";
 import { formatDate, statusBadgeClass } from "@/lib/format";
+import { StartTimerButton } from "@/components/timer/start-timer-button";
 import { toast } from "sonner";
 
 export const Route = createFileRoute("/clients/$clientId")({
@@ -79,6 +80,11 @@ function ClientDetail() {
         description={client.management_company || client.client_type.toUpperCase()}
         actions={
           <>
+            <StartTimerButton
+              variant="outline"
+              target={{ clientId: client.id, clientName: client.name }}
+              label="Start client timer"
+            />
             {canEdit && (
               <Button variant="outline" onClick={() => setEditOpen(true)}>
                 <Edit className="h-4 w-4 mr-2" /> Edit