diff --git a/packages/db/src/backup-lib.test.ts b/packages/db/src/backup-lib.test.ts
index 8c2fb32f8b..775226957b 100644
--- a/packages/db/src/backup-lib.test.ts
+++ b/packages/db/src/backup-lib.test.ts
@@ -75,6 +75,45 @@ describe("createBufferedTextFileWriter", () => {
 });
 
 describeEmbeddedPostgres("runDatabaseBackup", () => {
+  it(
+    "keeps the newest backup for each retained calendar month",
+    async () => {
+      const sourceConnectionString = await createTempDatabase();
+      const backupDir = createTempDir("paperclip-db-backup-retention-");
+      const realDateNow = Date.now;
+      Date.now = () => Date.UTC(2026, 2, 31, 12, 0, 0);
+
+      const janNewest = path.join(backupDir, "paperclip-test-2026-01-28T12-00-00.sql.gz");
+      const janOlder = path.join(backupDir, "paperclip-test-2026-01-10T12-00-00.sql.gz");
+      const decOld = path.join(backupDir, "paperclip-test-2025-12-15T12-00-00.sql.gz");
+
+      try {
+        fs.writeFileSync(janNewest, "jan-newest");
+        fs.writeFileSync(janOlder, "jan-older");
+        fs.writeFileSync(decOld, "dec-old");
+
+        fs.utimesSync(janNewest, new Date("2026-01-28T12:00:00Z"), new Date("2026-01-28T12:00:00Z"));
+        fs.utimesSync(janOlder, new Date("2026-01-10T12:00:00Z"), new Date("2026-01-10T12:00:00Z"));
+        fs.utimesSync(decOld, new Date("2025-12-15T12:00:00Z"), new Date("2025-12-15T12:00:00Z"));
+
+        const result = await runDatabaseBackup({
+          connectionString: sourceConnectionString,
+          backupDir,
+          retention: { dailyDays: 7, weeklyWeeks: 4, monthlyMonths: 2 },
+          filenamePrefix: "paperclip-test",
+        });
+
+        expect(result.prunedCount).toBe(2);
+        expect(fs.existsSync(janNewest)).toBe(true);
+        expect(fs.existsSync(janOlder)).toBe(false);
+        expect(fs.existsSync(decOld)).toBe(false);
+      } finally {
+        Date.now = realDateNow;
+      }
+    },
+    30_000,
+  );
+
   it(
     "backs up and restores large table payloads without materializing one giant string",
     async () => {
diff --git a/packages/db/src/backup-lib.ts b/packages/db/src/backup-lib.ts
index 41750e77ea..9e3760c758 100644
--- a/packages/db/src/backup-lib.ts
+++ b/packages/db/src/backup-lib.ts
@@ -104,7 +104,13 @@ function isoWeekKey(date: Date): string {
 }
 
 function monthKey(date: Date): string {
-  return `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, "0")}`;
+  return `${date.getUTCFullYear()}-${String(date.getUTCMonth() + 1).padStart(2, "0")}`;
+}
+
+function monthlyRetentionCutoff(nowMs: number, monthlyMonths: number): number {
+  const months = Math.max(1, monthlyMonths);
+  const now = new Date(nowMs);
+  return Date.UTC(now.getUTCFullYear(), now.getUTCMonth() - months, 1);
 }
 
 /**
@@ -120,7 +126,7 @@ function pruneOldBackups(backupDir: string, retention: BackupRetentionPolicy, fi
   const now = Date.now();
   const dailyCutoff = now - Math.max(1, retention.dailyDays) * 24 * 60 * 60 * 1000;
   const weeklyCutoff = now - Math.max(1, retention.weeklyWeeks) * 7 * 24 * 60 * 60 * 1000;
-  const monthlyCutoff = now - Math.max(1, retention.monthlyMonths) * 30 * 24 * 60 * 60 * 1000;
+  const monthlyCutoff = monthlyRetentionCutoff(now, retention.monthlyMonths);
 
   type BackupEntry = { name: string; fullPath: string; mtimeMs: number };
   const entries: BackupEntry[] = [];