diff --git a/app/api/counters/[id]/log/route.ts b/app/api/counters/[id]/log/route.ts
new file mode 100644
index 0000000..40d6d69
--- /dev/null
+++ b/app/api/counters/[id]/log/route.ts
@@ -0,0 +1,39 @@
+import { NextResponse } from 'next/server';
+import db from '@/lib/db';
+
+export const dynamic = 'force-dynamic';
+
+type Params = { params: Promise<{ id: string }> };
+
+export async function POST(request: Request, { params }: Params) {
+  const { id } = await params;
+  const { date, delta } = await request.json();
+
+  if (!date || typeof date !== 'string' || !/^\d{4}-\d{2}-\d{2}$/.test(date)) {
+    return NextResponse.json({ error: 'Invalid date' }, { status: 400 });
+  }
+  if (delta !== 1 && delta !== -1) {
+    return NextResponse.json({ error: 'Delta must be 1 or -1' }, { status: 400 });
+  }
+
+  const existing = db.prepare('SELECT * FROM counters WHERE id = ? AND archived_at IS NULL').get(Number(id)) as Record<string, unknown> | undefined;
+  if (!existing) return NextResponse.json({ error: 'Not found' }, { status: 404 });
+
+  // Store at noon local server time so SQLite's 'localtime' grouping matches the picked date
+  const createdAt = new Date(`${date}T12:00:00`).getTime();
+  if (isNaN(createdAt)) return NextResponse.json({ error: 'Invalid date' }, { status: 400 });
+
+  const result = db.transaction(() => {
+    const updated = db.prepare(
+      delta === 1
+        ? 'UPDATE counters SET value = value + 1 WHERE id = ? RETURNING *'
+        : 'UPDATE counters SET value = value - 1 WHERE id = ? AND value > 0 RETURNING *'
+    ).get(Number(id)) as Record<string, unknown> | undefined;
+    if (!updated) return null;
+    db.prepare('INSERT INTO events (counter_id, delta, created_at) VALUES (?, ?, ?)').run(Number(id), delta, createdAt);
+    return updated;
+  })();
+
+  if (!result) return NextResponse.json({ error: 'Counter cannot go below 0' }, { status: 400 });
+  return NextResponse.json(result);
+}
diff --git a/app/components/CounterDetailModal.tsx b/app/components/CounterDetailModal.tsx
index 151ca3f..9576773 100644
--- a/app/components/CounterDetailModal.tsx
+++ b/app/components/CounterDetailModal.tsx
@@ -15,6 +15,7 @@ interface Props {
   counterId: number | null;
   cache?: Map<number, HistoryData>;
   onClose: () => void;
+  onLog?: (counter: Counter) => void;
 }
 
 function toLocalDateStr(d: Date): string {
@@ -47,12 +48,18 @@ function computeLongestStreak(sortedDates: string[]): number {
   return best;
 }
 
-export default function CounterDetailModal({ counterId, cache, onClose }: Props) {
+export default function CounterDetailModal({ counterId, cache, onClose, onLog }: Props) {
   const [data, setData] = useState<HistoryData | null>(null);
   const [loading, setLoading] = useState(false);
+  const [logDate, setLogDate] = useState(() => { const d = new Date(); d.setDate(d.getDate() - 1); return toLocalDateStr(d); });
+  const [logging, setLogging] = useState(false);
+  const [logError, setLogError] = useState('');
 
   useEffect(() => {
     if (!counterId) return;
+    const yesterday = new Date(); yesterday.setDate(yesterday.getDate() - 1);
+    setLogDate(toLocalDateStr(yesterday));
+    setLogError('');
     const cached = cache?.get(counterId);
     if (cached) { setData(cached); setLoading(false); return; }
     setData(null);
@@ -70,6 +77,33 @@ export default function CounterDetailModal({ counterId, cache, onClose }: Props)
     return () => window.removeEventListener('keydown', handler);
   }, [counterId, onClose]);
 
+  async function handleLog(delta: number) {
+    if (!counterId || !logDate) return;
+    setLogging(true);
+    setLogError('');
+    try {
+      const res = await fetch(`/api/counters/${counterId}/log`, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ date: logDate, delta }),
+      });
+      if (!res.ok) {
+        const body = await res.json();
+        setLogError(body.error ?? 'Failed to log entry');
+        return;
+      }
+      const updated: Counter = await res.json();
+      cache?.delete(counterId);
+      const histRes = await fetch(`/api/counters/${counterId}/history`);
+      const d: HistoryData = await histRes.json();
+      cache?.set(counterId, d);
+      setData(d);
+      onLog?.(updated);
+    } finally {
+      setLogging(false);
+    }
+  }
+
   if (!counterId) return null;
 
   const dateSet = new Set(data?.dailyActivity.map(d => d.date) ?? []);
@@ -165,6 +199,42 @@ export default function CounterDetailModal({ counterId, cache, onClose }: Props)
                   </div>
                 </>
               )}
+
+              {/* Log past entry */}
+              <div className="border-t border-ctp-surface1 pt-4">
+                <h3 className="text-sm font-bold text-ctp-text mb-3">Log past entry</h3>
+                <div className="flex items-center gap-2 flex-wrap">
+                  <input
+                    type="date"
+                    value={logDate}
+                    max={toLocalDateStr(new Date())}
+                    onChange={e => setLogDate(e.target.value)}
+                    className="rounded-lg border border-ctp-surface1 bg-ctp-surface0 text-ctp-text px-3 py-2 text-sm focus:outline-none focus:ring-2 focus:ring-ctp-mauve"
+                  />
+                  <button
+                    onClick={() => handleLog(-1)}
+                    disabled={logging || !logDate || (data?.dailyActivity.find(d => d.date === logDate)?.value ?? 0) <= 0}
+                    aria-label="Log decrement"
+                    className="w-10 h-10 flex items-center justify-center rounded-xl bg-ctp-surface1 text-ctp-red hover:bg-ctp-red/20 active:bg-ctp-red/30 disabled:opacity-40 transition-colors"
+                  >
+                    <svg xmlns="http://www.w3.org/2000/svg" className="w-4 h-4" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2.5" strokeLinecap="round">
+                      <line x1="5" y1="12" x2="19" y2="12" />
+                    </svg>
+                  </button>
+                  <button
+                    onClick={() => handleLog(1)}
+                    disabled={logging || !logDate}
+                    aria-label="Log increment"
+                    className="w-10 h-10 flex items-center justify-center rounded-xl bg-ctp-surface1 text-ctp-green hover:bg-ctp-green/20 active:bg-ctp-green/30 disabled:opacity-40 transition-colors"
+                  >
+                    <svg xmlns="http://www.w3.org/2000/svg" className="w-4 h-4" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2.5" strokeLinecap="round">
+                      <line x1="12" y1="5" x2="12" y2="19" />
+                      <line x1="5" y1="12" x2="19" y2="12" />
+                    </svg>
+                  </button>
+                  {logError && <p className="text-xs text-ctp-red">{logError}</p>}
+                </div>
+              </div>
             </>
           )}
         </div>
diff --git a/app/page.tsx b/app/page.tsx
index f47abb2..d94a01c 100644
--- a/app/page.tsx
+++ b/app/page.tsx
@@ -495,6 +495,10 @@ export default function Home() {
         counterId={historyCounterId}
         cache={historyCache.current}
         onClose={() => setHistoryCounterId(null)}
+        onLog={updated => {
+          setCounters(prev => prev.map(c => c.id === updated.id ? { ...c, value: updated.value } : c));
+          setArchivedCounters(prev => prev.map(c => c.id === updated.id ? { ...c, value: updated.value } : c));
+        }}
       />
     </main>
   );