diff --git a/src/lib/measurements/rollups.ts b/src/lib/measurements/rollups.ts
index 41c23336e..897789a30 100644
--- a/src/lib/measurements/rollups.ts
+++ b/src/lib/measurements/rollups.ts
@@ -31,6 +31,7 @@
 import { prisma } from "@/lib/db";
 import { getGlobalBoss } from "@/lib/jobs/boss-instance";
 import { annotate } from "@/lib/logging/context";
+import { startOfUtcDay } from "@/lib/tz/start-of-utc-day";
 import type {
   MeasurementType,
   RollupGranularity,
@@ -518,17 +519,6 @@ async function persistRollupRows(
   return touched;
 }
 
-/**
- * Truncate a `Date` to the start of its UTC day. The rollup table
- * stores TIMESTAMPTZ values so we anchor on UTC to keep the bucket
- * key deterministic across timezone-shifting consumers.
- */
-function startOfUtcDay(d: Date): Date {
-  return new Date(
-    Date.UTC(d.getUTCFullYear(), d.getUTCMonth(), d.getUTCDate(), 0, 0, 0, 0),
-  );
-}
-
 /**
  * Span of the bucket containing `measuredAt` at the given granularity.
  * Postgres `date_trunc` is the canonical truncator; we mirror its
diff --git a/src/lib/mood/rollups.ts b/src/lib/mood/rollups.ts
index 4601db8e8..bb7df0069 100644
--- a/src/lib/mood/rollups.ts
+++ b/src/lib/mood/rollups.ts
@@ -40,6 +40,7 @@
 import { prisma } from "@/lib/db";
 import { getGlobalBoss } from "@/lib/jobs/boss-instance";
 import { annotate } from "@/lib/logging/context";
+import { startOfUtcDay } from "@/lib/tz/start-of-utc-day";
 import type {
   Prisma,
   PrismaClient,
@@ -611,18 +612,6 @@ async function persistMoodRollupRows(
   return touched;
 }
 
-/**
- * Truncate a `Date` to the start of its UTC day. The rollup table
- * stores TIMESTAMPTZ values so we anchor on UTC to keep the bucket
- * key deterministic across timezone-shifting consumers — same
- * convention as `measurement-rollups`.
- */
-function startOfUtcDay(d: Date): Date {
-  return new Date(
-    Date.UTC(d.getUTCFullYear(), d.getUTCMonth(), d.getUTCDate(), 0, 0, 0, 0),
-  );
-}
-
 /**
  * Span of the bucket containing `moodLoggedAt` at the given
  * granularity. Mirrors `measurement-rollups` so the WEEK / MONTH /
diff --git a/src/lib/tz/start-of-utc-day.ts b/src/lib/tz/start-of-utc-day.ts
new file mode 100644
index 000000000..eb9cc53be
--- /dev/null
+++ b/src/lib/tz/start-of-utc-day.ts
@@ -0,0 +1,20 @@
+/**
+ * UTC-anchored start-of-day truncation shared by the measurement +
+ * mood rollup writers.
+ *
+ * Both rollup tables store TIMESTAMPTZ bucket starts. Anchoring on the
+ * UTC calendar day (rather than the user's local day) keeps the bucket
+ * key deterministic across timezone-shifting consumers — the same
+ * convention `date_trunc('day', measured_at AT TIME ZONE 'UTC')` would
+ * produce on the read side. The display surfaces re-bucket into the
+ * user's timezone via `userDayKey()` from `./format`.
+ *
+ * v1.4.40 W-GHOSTS consolidated two byte-identical local copies
+ * (`src/lib/measurements/rollups.ts` + `src/lib/mood/rollups.ts`) into
+ * this module so future writers cannot drift independently.
+ */
+export function startOfUtcDay(d: Date): Date {
+  return new Date(
+    Date.UTC(d.getUTCFullYear(), d.getUTCMonth(), d.getUTCDate(), 0, 0, 0, 0),
+  );
+}