diff --git a/src/components/app-shell.tsx b/src/components/app-shell.tsx
index 5087d5e..a30ff9d 100644
--- a/src/components/app-shell.tsx
+++ b/src/components/app-shell.tsx
@@ -19,6 +19,7 @@ import { cn } from "@/lib/utils";
 import type { ReactNode } from "react";
 import { useBubbleCounts } from "@/lib/use-bubble-counts";
 import { NotificationBell } from "@/components/notifications/notification-bell";
+import { DateCalculator } from "@/components/date-calculator";
 
 function initials(name: string, email: string) {
   const src = (name || email || "").trim();
@@ -102,6 +103,7 @@ export function AppShell({ children }: { children: ReactNode }) {
             <div className="font-serif text-lg truncate">Stage Law Firm, PLLC</div>
             <div className="text-[10px] uppercase tracking-widest text-sidebar-foreground/60">Law Firm Suite</div>
           </div>
+          <DateCalculator />
           <NotificationBell />
         </div>
 
@@ -168,6 +170,7 @@ export function AppShell({ children }: { children: ReactNode }) {
           <span className="font-serif text-lg">Counsel</span>
         </Link>
         <div className="flex items-center gap-2">
+          <DateCalculator />
           <NotificationBell />
           <Button variant="ghost" size="sm" onClick={handleSignOut} className="text-sidebar-foreground">
             <LogOut className="h-4 w-4" />
diff --git a/src/components/date-calculator.tsx b/src/components/date-calculator.tsx
new file mode 100644
index 0000000..5de627c
--- /dev/null
+++ b/src/components/date-calculator.tsx
@@ -0,0 +1,245 @@
+import { useMemo, useState } from "react";
+import { Popover, PopoverContent, PopoverTrigger } from "@/components/ui/popover";
+import { Button } from "@/components/ui/button";
+import { Input } from "@/components/ui/input";
+import { Label } from "@/components/ui/label";
+import { Calendar } from "@/components/ui/calendar";
+import {
+  Select,
+  SelectContent,
+  SelectItem,
+  SelectTrigger,
+  SelectValue,
+} from "@/components/ui/select";
+import { Tabs, TabsList, TabsTrigger, TabsContent } from "@/components/ui/tabs";
+import { CalendarClock, CalendarIcon } from "lucide-react";
+import { cn } from "@/lib/utils";
+import { format } from "date-fns";
+
+type Unit = "days" | "weeks" | "months" | "years";
+
+function addUnits(base: Date, amount: number, unit: Unit, businessOnly: boolean): Date {
+  const d = new Date(base);
+  if (unit === "days") {
+    if (!businessOnly) {
+      d.setDate(d.getDate() + amount);
+      return d;
+    }
+    let remaining = Math.abs(amount);
+    const step = amount >= 0 ? 1 : -1;
+    while (remaining > 0) {
+      d.setDate(d.getDate() + step);
+      const day = d.getDay();
+      if (day !== 0 && day !== 6) remaining--;
+    }
+    return d;
+  }
+  if (unit === "weeks") {
+    d.setDate(d.getDate() + amount * 7);
+    return d;
+  }
+  if (unit === "months") {
+    d.setMonth(d.getMonth() + amount);
+    return d;
+  }
+  d.setFullYear(d.getFullYear() + amount);
+  return d;
+}
+
+function daysBetween(a: Date, b: Date, businessOnly: boolean) {
+  const start = new Date(a);
+  start.setHours(0, 0, 0, 0);
+  const end = new Date(b);
+  end.setHours(0, 0, 0, 0);
+  const sign = end >= start ? 1 : -1;
+  if (!businessOnly) {
+    return Math.round((end.getTime() - start.getTime()) / 86400000);
+  }
+  let count = 0;
+  const cur = new Date(start);
+  while (cur.getTime() !== end.getTime()) {
+    cur.setDate(cur.getDate() + sign);
+    const day = cur.getDay();
+    if (day !== 0 && day !== 6) count += sign;
+  }
+  return count;
+}
+
+export function DateCalculator({ trigger }: { trigger?: React.ReactNode }) {
+  const [open, setOpen] = useState(false);
+
+  // Add/subtract state
+  const [baseDate, setBaseDate] = useState<Date>(new Date());
+  const [amount, setAmount] = useState<string>("30");
+  const [unit, setUnit] = useState<Unit>("days");
+  const [businessOnly, setBusinessOnly] = useState(false);
+  const [direction, setDirection] = useState<"add" | "sub">("add");
+
+  // Diff state
+  const [fromDate, setFromDate] = useState<Date>(new Date());
+  const [toDate, setToDate] = useState<Date>(() => {
+    const d = new Date();
+    d.setDate(d.getDate() + 30);
+    return d;
+  });
+  const [diffBusinessOnly, setDiffBusinessOnly] = useState(false);
+
+  const result = useMemo(() => {
+    const n = parseInt(amount || "0", 10);
+    if (Number.isNaN(n)) return null;
+    return addUnits(baseDate, direction === "add" ? n : -n, unit, businessOnly);
+  }, [baseDate, amount, unit, direction, businessOnly]);
+
+  const diff = useMemo(() => daysBetween(fromDate, toDate, diffBusinessOnly), [
+    fromDate,
+    toDate,
+    diffBusinessOnly,
+  ]);
+
+  return (
+    <Popover open={open} onOpenChange={setOpen}>
+      <PopoverTrigger asChild>
+        {trigger ?? (
+          <Button
+            variant="ghost"
+            size="icon"
+            className="h-8 w-8 text-sidebar-foreground hover:bg-sidebar-accent/60 hover:text-sidebar-accent-foreground"
+            aria-label="Date calculator"
+          >
+            <CalendarClock className="h-4 w-4" />
+          </Button>
+        )}
+      </PopoverTrigger>
+      <PopoverContent align="end" className="w-[340px] p-0">
+        <div className="px-4 py-3 border-b">
+          <div className="font-serif text-sm">Date calculator</div>
+          <div className="text-[11px] text-muted-foreground">
+            Add/subtract time or count days between dates.
+          </div>
+        </div>
+        <Tabs defaultValue="add" className="w-full">
+          <TabsList className="grid grid-cols-2 m-3">
+            <TabsTrigger value="add">Add / subtract</TabsTrigger>
+            <TabsTrigger value="diff">Difference</TabsTrigger>
+          </TabsList>
+
+          <TabsContent value="add" className="px-4 pb-4 space-y-3 mt-0">
+            <div className="space-y-1.5">
+              <Label className="text-xs">Start date</Label>
+              <DatePopover value={baseDate} onChange={setBaseDate} />
+            </div>
+            <div className="grid grid-cols-3 gap-2">
+              <div className="space-y-1.5">
+                <Label className="text-xs">Op</Label>
+                <Select value={direction} onValueChange={(v) => setDirection(v as "add" | "sub")}>
+                  <SelectTrigger className="h-9"><SelectValue /></SelectTrigger>
+                  <SelectContent>
+                    <SelectItem value="add">+</SelectItem>
+                    <SelectItem value="sub">−</SelectItem>
+                  </SelectContent>
+                </Select>
+              </div>
+              <div className="space-y-1.5">
+                <Label className="text-xs">Amount</Label>
+                <Input
+                  type="number"
+                  value={amount}
+                  onChange={(e) => setAmount(e.target.value)}
+                  className="h-9"
+                />
+              </div>
+              <div className="space-y-1.5">
+                <Label className="text-xs">Unit</Label>
+                <Select value={unit} onValueChange={(v) => setUnit(v as Unit)}>
+                  <SelectTrigger className="h-9"><SelectValue /></SelectTrigger>
+                  <SelectContent>
+                    <SelectItem value="days">Days</SelectItem>
+                    <SelectItem value="weeks">Weeks</SelectItem>
+                    <SelectItem value="months">Months</SelectItem>
+                    <SelectItem value="years">Years</SelectItem>
+                  </SelectContent>
+                </Select>
+              </div>
+            </div>
+            {unit === "days" && (
+              <label className="flex items-center gap-2 text-xs text-muted-foreground cursor-pointer">
+                <input
+                  type="checkbox"
+                  checked={businessOnly}
+                  onChange={(e) => setBusinessOnly(e.target.checked)}
+                  className="rounded border-input"
+                />
+                Business days only (Mon–Fri)
+              </label>
+            )}
+            <div className="rounded-md border bg-muted/40 px-3 py-2.5">
+              <div className="text-[10px] uppercase tracking-wider text-muted-foreground">Result</div>
+              <div className="font-serif text-base mt-0.5">
+                {result ? format(result, "EEEE, MMM d, yyyy") : "—"}
+              </div>
+            </div>
+          </TabsContent>
+
+          <TabsContent value="diff" className="px-4 pb-4 space-y-3 mt-0">
+            <div className="space-y-1.5">
+              <Label className="text-xs">From</Label>
+              <DatePopover value={fromDate} onChange={setFromDate} />
+            </div>
+            <div className="space-y-1.5">
+              <Label className="text-xs">To</Label>
+              <DatePopover value={toDate} onChange={setToDate} />
+            </div>
+            <label className="flex items-center gap-2 text-xs text-muted-foreground cursor-pointer">
+              <input
+                type="checkbox"
+                checked={diffBusinessOnly}
+                onChange={(e) => setDiffBusinessOnly(e.target.checked)}
+                className="rounded border-input"
+              />
+              Business days only (Mon–Fri)
+            </label>
+            <div className="rounded-md border bg-muted/40 px-3 py-2.5">
+              <div className="text-[10px] uppercase tracking-wider text-muted-foreground">
+                {diffBusinessOnly ? "Business days" : "Calendar days"}
+              </div>
+              <div className="font-serif text-base mt-0.5">
+                {diff} day{Math.abs(diff) === 1 ? "" : "s"}
+              </div>
+            </div>
+          </TabsContent>
+        </Tabs>
+      </PopoverContent>
+    </Popover>
+  );
+}
+
+function DatePopover({ value, onChange }: { value: Date; onChange: (d: Date) => void }) {
+  const [open, setOpen] = useState(false);
+  return (
+    <Popover open={open} onOpenChange={setOpen}>
+      <PopoverTrigger asChild>
+        <Button
+          variant="outline"
+          className={cn("w-full justify-start text-left font-normal h-9")}
+        >
+          <CalendarIcon className="mr-2 h-4 w-4 opacity-60" />
+          {format(value, "PPP")}
+        </Button>
+      </PopoverTrigger>
+      <PopoverContent className="w-auto p-0" align="start">
+        <Calendar
+          mode="single"
+          selected={value}
+          onSelect={(d) => {
+            if (d) {
+              onChange(d);
+              setOpen(false);
+            }
+          }}
+          initialFocus
+          className={cn("p-3 pointer-events-auto")}
+        />
+      </PopoverContent>
+    </Popover>
+  );
+}