diff --git a/src/lib/jobs/queue-runtime.ts b/src/lib/jobs/queue-runtime.ts
index 3c3d007ff..3bc085fd0 100644
--- a/src/lib/jobs/queue-runtime.ts
+++ b/src/lib/jobs/queue-runtime.ts
@@ -190,7 +190,9 @@ export const QUEUE_RUNTIME: Readonly<Record<string, QueueRuntime>> = {
   "daily-briefing": short("A light cohort scan every fifteen minutes."),
 
   // ── Rollups ────────────────────────────────────────────────────────
-  "rollup-recompute": short("One bucket."),
+  "rollup-recompute": short(
+    "One type at one granularity over one bucket or a run of adjacent buckets, inside the five-year fold window.",
+  ),
   "rollup-full-backfill": {
     runtime: "long",
     expireInSeconds: ROLLUP_FULL_BACKFILL_EXPIRE_SECONDS,
diff --git a/src/lib/rollups/measurement-rollups.ts b/src/lib/rollups/measurement-rollups.ts
index 0a9eae21b..a5277687f 100644
--- a/src/lib/rollups/measurement-rollups.ts
+++ b/src/lib/rollups/measurement-rollups.ts
@@ -362,8 +362,19 @@ export async function recomputeBucketsForMeasurement(
   );
 }
 
+/**
+ * UTC wall-clock literals for a `timestamp[]` parameter. `measured_at` and
+ * `bucket_start` are naive UTC timestamps, so the instants they are compared
+ * with must be too: a `timestamptz[]` parameter is converted through the
+ * session time zone, and in any zone but UTC neither the day filter nor the
+ * keyed delete matched a single day, so a refold left its buckets stale.
+ */
+function utcStamps(dates: readonly Date[]): string[] {
+  return dates.map((d) => d.toISOString().replace("Z", ""));
+}
+
 /** Keys one set-based DAY refold carries; bounds the statement's arrays. */
-const KEYED_REFOLD_CHUNK = 2_000;
+const KEYED_REFOLD_CHUNK = 500;
 
 /**
  * The set-based sibling of {@link recomputeBucketsForMeasurement}, for a write
@@ -381,7 +392,17 @@ export async function recomputeBucketsForKeys(
   userId: string,
   rows: Array<{ type: MeasurementType; measuredAt: Date }>,
 ): Promise<void> {
-  const keys = collapseToTypeDayKeys(rows);
+  // Sorted on (type, day), the order every upsert of the tier takes its row
+  // locks in (`persistRollupRows` sorts the same way), and chunked small, so
+  // a keyed refold racing the full fold or another refold holds a short,
+  // deterministic slice and waits rather than interleaving lock orders.
+  const keys = collapseToTypeDayKeys(rows).sort((a, b) =>
+    a.type !== b.type
+      ? a.type < b.type
+        ? -1
+        : 1
+      : a.measuredAt.getTime() - b.measuredAt.getTime(),
+  );
   for (let i = 0; i < keys.length; i += KEYED_REFOLD_CHUNK) {
     await refoldDayBuckets(userId, keys.slice(i, i + KEYED_REFOLD_CHUNK));
   }
@@ -441,11 +462,11 @@ async function refoldDayBuckets(
             AND granularity = 'DAY'::"measurement_rollup_granularity"
             AND (type, bucket_start) IN (
               SELECT k.t::"measurement_type", k.b
-                FROM unnest($2::text[], $3::timestamptz[]) AS k(t, b)
+                FROM unnest($2::text[], $3::timestamp[]) AS k(t, b)
             )`,
         userId,
         keys.map((k) => k.type),
-        keys.map((k) => k.measuredAt.toISOString()),
+        utcStamps(keys.map((k) => k.measuredAt)),
       );
       for (let i = 0; i < ordered.length; i += 500) {
         const { sql, params } = buildRollupUpsert(
@@ -642,9 +663,9 @@ async function runRollupAggregate(
   if (input.types && input.types.length === 0) return [];
   // Bound as the fourth parameter, never spliced.
   const dayFilter = input.days
-    ? `AND date_trunc('day', m."measured_at") = ANY($4::timestamptz[])`
+    ? `AND date_trunc('day', m."measured_at") = ANY($4::timestamp[])`
     : "";
-  const dayParams = input.days ? [input.days.map((d) => d.toISOString())] : [];
+  const dayParams = input.days ? [utcStamps(input.days)] : [];
 
   // Postgres rejects `date_trunc($1::text, m."measured_at")` in a
   // GROUP BY because the planner sees the parameterised expression
@@ -857,7 +878,7 @@ async function applyHourlyMeanDay(
           AND m."deleted_at" IS NULL
           ${
             input.days
-              ? `AND date_trunc('day', m."measured_at") = ANY($4::timestamptz[])`
+              ? `AND date_trunc('day', m."measured_at") = ANY($4::timestamp[])`
               : ""
           }
         GROUP BY m."type", m."source", date_trunc('hour', m."measured_at")
@@ -869,7 +890,7 @@ async function applyHourlyMeanDay(
     input.userId,
     input.from,
     input.to,
-    ...(input.days ? [input.days.map((d) => d.toISOString())] : []),
+    ...(input.days ? [utcStamps(input.days)] : []),
   );
   const byKey = new Map(
     means.map((m) => [
diff --git a/tests/integration/rollup-keyed-refold-session-tz.test.ts b/tests/integration/rollup-keyed-refold-session-tz.test.ts
new file mode 100644
index 000000000..30e88bb3c
--- /dev/null
+++ b/tests/integration/rollup-keyed-refold-session-tz.test.ts
@@ -0,0 +1,76 @@
+/**
+ * The keyed DAY refold does not depend on the session time zone.
+ *
+ * `measured_at` and `bucket_start` are naive UTC timestamps. The refold's day
+ * filter and its delete compared them with a `timestamptz[]` parameter, which
+ * Postgres converts through the session time zone: outside UTC neither matched
+ * a single day, so the refold silently left every touched bucket stale. The
+ * app pins its sessions to UTC, so this runs the pool in another zone on
+ * purpose.
+ */
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("@/lib/db-session", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("@/lib/db-session")>();
+  return { ...actual, TIME_ZONE_OPTION: "-c TimeZone=Pacific/Auckland" };
+});
+
+vi.mock("@/lib/db-compat", () => ({
+  ensureDbCompatibility: vi.fn().mockResolvedValue(undefined),
+}));
+
+import { getPrismaClient, truncateAllTables } from "./setup";
+import { prisma } from "@/lib/db";
+import {
+  recomputeBucketsForKeys,
+  recomputeBucketsForMeasurement,
+} from "@/lib/rollups/measurement-rollups";
+
+beforeEach(async () => {
+  await truncateAllTables(getPrismaClient());
+});
+
+describe("keyed DAY refold in a non-UTC session", () => {
+  it("folds a new reading into its day", async () => {
+    const [{ zone }] = await prisma.$queryRaw<Array<{ zone: string }>>`
+      SELECT current_setting('TimeZone') AS zone
+    `;
+    expect(zone).toBe("Pacific/Auckland");
+
+    const user = await getPrismaClient().user.create({
+      data: { username: "tz-refold", email: "tz-refold@example.test" },
+    });
+    const day = new Date("2026-05-10T00:00:00.000Z");
+    await getPrismaClient().measurement.createMany({
+      data: [7, 19].map((hour, i) => ({
+        userId: user.id,
+        type: "WEIGHT" as const,
+        value: 80 + i,
+        unit: "kg",
+        measuredAt: new Date(day.getTime() + hour * 3_600_000),
+      })),
+    });
+    await recomputeBucketsForMeasurement(user.id, "WEIGHT", day);
+    // A third reading lands; the refold has to fold it in.
+    await getPrismaClient().measurement.create({
+      data: {
+        userId: user.id,
+        type: "WEIGHT",
+        value: 82,
+        unit: "kg",
+        measuredAt: new Date(day.getTime() + 21 * 3_600_000),
+      },
+    });
+
+    await recomputeBucketsForKeys(user.id, [
+      { type: "WEIGHT", measuredAt: new Date(day.getTime() + 7 * 3_600_000) },
+    ]);
+
+    const rows = await getPrismaClient().measurementRollup.findMany({
+      where: { userId: user.id, granularity: "DAY" },
+    });
+    expect(rows).toHaveLength(1);
+    expect(rows[0]!.bucketStart.toISOString()).toBe(day.toISOString());
+    expect(rows[0]!.count).toBe(3);
+  });
+});