diff --git a/src/lib/pdf/parseDeviceInstant.ts b/src/lib/pdf/parseDeviceInstant.ts
index beb9a007..30bd8198 100644
--- a/src/lib/pdf/parseDeviceInstant.ts
+++ b/src/lib/pdf/parseDeviceInstant.ts
@@ -6,11 +6,11 @@ export const timestampHasExplicitOffset = (value: string): boolean =>
 	tzOffsetPattern.test(value.trim());
 
 /**
- * Normalize device timestamps into the requested timezone. When a timestamp
- * string lacks an explicit offset we treat it as already representing the
- * user's chosen timezone and simply attach that zone so the rendering is
- * independent of the host machine's locale (critical on Vercel where the
- * runtime is UTC).
+ * Normalize device timestamps into the requested timezone. Strings without an
+ * explicit offset are assumed to be stored in UTC (Postgres default) and are
+ * therefore converted from UTC into the caller's timezone so that report data
+ * always represents the user's day (JST, etc.) regardless of where the server
+ * runs.
  */
 export function parseDeviceInstant(input: string | Date, tz: string): DateTime {
 	if (input instanceof Date) {
@@ -33,8 +33,8 @@ export function parseDeviceInstant(input: string | Date, tz: string): DateTime {
 		return dt.setZone(tz);
 	}
 
-	let dt = DateTime.fromISO(value, { zone: tz });
-	if (!dt.isValid) dt = DateTime.fromSQL(value, { zone: tz });
-	if (!dt.isValid) dt = DateTime.fromRFC2822(value, { zone: tz });
+	let dt = DateTime.fromISO(value, { zone: 'utc' });
+	if (!dt.isValid) dt = DateTime.fromSQL(value, { zone: 'utc' });
+	if (!dt.isValid) dt = DateTime.fromRFC2822(value, { zone: 'utc' });
 	return dt.setZone(tz);
 }
diff --git a/src/lib/services/DeviceDataService.ts b/src/lib/services/DeviceDataService.ts
index 45aa836a..009a3ba1 100644
--- a/src/lib/services/DeviceDataService.ts
+++ b/src/lib/services/DeviceDataService.ts
@@ -690,11 +690,16 @@ export class DeviceDataService implements IDeviceDataService {
 			const asIso = DateTime.fromJSDate(utcTimestamp, { zone: 'utc' }).setZone(timezone).toISO();
 			return asIso ?? utcTimestamp.toISOString();
 		}
+
+		const normalized =
+			typeof utcTimestamp === 'string' ? utcTimestamp.trim() : String(utcTimestamp);
+		if (!normalized) {
+			return normalized;
+		}
 		if (timezone === 'UTC') {
-			return typeof utcTimestamp === 'string' ? utcTimestamp : String(utcTimestamp);
+			return normalized;
 		}
 
-		const normalized = utcTimestamp.trim();
 		const hasOffset = this.timezoneOffsetPattern.test(normalized);
 		let dt = DateTime.invalid('unparsed');
 
@@ -702,22 +707,18 @@ export class DeviceDataService implements IDeviceDataService {
 			dt = DateTime.fromISO(normalized, { setZone: true });
 			if (!dt.isValid) dt = DateTime.fromSQL(normalized, { setZone: true });
 			if (!dt.isValid) dt = DateTime.fromRFC2822(normalized, { setZone: true });
-			if (!dt.isValid) {
-				console.warn(`Failed to parse timestamp with offset: ${utcTimestamp}`);
-				return normalized;
-			}
-			const converted = dt.setZone(timezone).toISO();
-			return converted ?? normalized;
+		} else {
+			dt = DateTime.fromISO(normalized, { zone: 'utc' });
+			if (!dt.isValid) dt = DateTime.fromSQL(normalized, { zone: 'utc' });
+			if (!dt.isValid) dt = DateTime.fromRFC2822(normalized, { zone: 'utc' });
 		}
 
-		dt = DateTime.fromISO(normalized, { zone: timezone });
-		if (!dt.isValid) dt = DateTime.fromSQL(normalized, { zone: timezone });
-		if (!dt.isValid) dt = DateTime.fromRFC2822(normalized, { zone: timezone });
 		if (!dt.isValid) {
-			console.warn(`Failed to interpret timestamp without offset for ${timezone}: ${utcTimestamp}`);
+			console.warn(`Failed to parse timestamp: ${utcTimestamp}`);
 			return normalized;
 		}
-		return dt.toISO() ?? normalized;
+
+		return dt.setZone(timezone).toISO() ?? normalized;
 	}
 
 	/**
diff --git a/src/lib/tests/ParseDeviceInstant.test.ts b/src/lib/tests/ParseDeviceInstant.test.ts
index c11784f4..4a2f790d 100644
--- a/src/lib/tests/ParseDeviceInstant.test.ts
+++ b/src/lib/tests/ParseDeviceInstant.test.ts
@@ -2,9 +2,9 @@ import { describe, expect, it } from 'vitest';
 import { parseDeviceInstant } from '$lib/pdf/parseDeviceInstant';
 
 describe('parseDeviceInstant', () => {
-	it('treats offset-less ISO timestamps as local values in the requested zone', () => {
+	it('treats offset-less ISO timestamps as UTC then converts to the requested zone', () => {
 		const dt = parseDeviceInstant('2025-11-08T15:00:00', 'Asia/Tokyo');
-		expect(dt.toISO()).toBe('2025-11-08T15:00:00.000+09:00');
+		expect(dt.toISO()).toBe('2025-11-09T00:00:00.000+09:00');
 	});
 
 	it('respects explicit offsets on the source timestamp', () => {