diff --git a/src/repositories/chatRepository.ts b/src/repositories/chatRepository.ts
index 7b469e7..09d09c0 100644
--- a/src/repositories/chatRepository.ts
+++ b/src/repositories/chatRepository.ts
@@ -15,7 +15,7 @@ import type {
   NavAction,
 } from "../types/chat";
 import type { TeamCode } from "../types/panit";
-import { addDaysKst, type DateString } from "../types/dateString";
+import { addDays, type DateString } from "../types/dateString";
 
 /**
  * AI 채팅 저장소(§4) — Firestore/RTDB 접근 전담.
@@ -214,7 +214,7 @@ export async function reserveRequestTx(params: ReserveParams): Promise<ReserveOu
 
 /** KST 자정 리셋 시각(다음 날 00:00 KST, ISO 8601). */
 export function kstResetAt(date: DateString): string {
-  return `${addDaysKst(date, 1)}T00:00:00+09:00`;
+  return `${addDays(date, 1)}T00:00:00+09:00`;
 }
 
 // ── 차감 복원(§3.1 복원 규칙) ──
diff --git a/src/scheduled/dailyArchive.ts b/src/scheduled/dailyArchive.ts
index f2a4798..6a26b92 100644
--- a/src/scheduled/dailyArchive.ts
+++ b/src/scheduled/dailyArchive.ts
@@ -14,7 +14,7 @@ import { deleteUserVoteGame } from "../repositories/voteRepository";
 import { processGameEndWithGame } from "../services/gameResultService";
 import type { RankSnapshot, VoteHistoryDoc } from "../types/panit";
 import {
-  dayOfWeekKst,
+  dayOfWeek,
   daysAgoKst,
   type DateString,
 } from "../types/dateString";
@@ -155,7 +155,7 @@ export async function runDailyArchive(
     }
 
     // 일요일(dow=0) 아카이브/판정이 끝난 시점에 주간 마스터 티켓 지급 판단.
-    if (dayOfWeekKst(date) === 0) {
+    if (dayOfWeek(date) === 0) {
       for (const uid of judgedUids) {
         try {
           await judgeWeekIfNeeded(uid, date);
diff --git a/src/services/attendanceService.ts b/src/services/attendanceService.ts
index fe13dad..78bde04 100644
--- a/src/services/attendanceService.ts
+++ b/src/services/attendanceService.ts
@@ -11,7 +11,7 @@ import {
   type PointAward,
 } from "../types/panit";
 import {
-  dayOfWeekKst,
+  dayOfWeek,
   isMonthString,
   lastDayOfMonth,
   monthOf,
@@ -190,7 +190,7 @@ export async function checkIn(
 
     // 2) weekly_bonus: 출석일이 일요일 + 그 주 월~토 모두 출석
     if (
-      dayOfWeekKst(today) === 0 &&
+      dayOfWeek(today) === 0 &&
       isPriorSixDaysAttended(newDays, todayDay)
     ) {
       runningBalance += WEEKLY_BONUS_AMOUNT;
diff --git a/src/services/chatToolService.ts b/src/services/chatToolService.ts
index ce02959..edfdc5b 100644
--- a/src/services/chatToolService.ts
+++ b/src/services/chatToolService.ts
@@ -14,7 +14,7 @@ import type { PlayerRecord } from "../kbo/player/common";
 import type { TeamRank } from "../kbo/team-rank";
 import type { ScheduleGame } from "../types/kbo";
 import { TeamCode, type AttendanceMonthDoc, type VoteHistoryDoc } from "../types/panit";
-import { addDaysKst, type DateString } from "../types/dateString";
+import { addDays, type DateString } from "../types/dateString";
 
 /**
  * 도구 이름 → 사람이 읽을 라벨. 클라이언트가 "🔍 순위 조회" 같은 칩으로 표시하기 위함.
@@ -81,10 +81,10 @@ export function resolveToolDate(raw: unknown, today: DateString): DateString | n
   if (typeof raw !== "string" || raw.trim() === "") return today;
   const s = raw.trim().toLowerCase();
   if (s === "today" || s === "오늘") return today;
-  if (s === "yesterday" || s === "어제") return addDaysKst(today, -1);
-  if (s === "그저께" || s === "그제" || s === "엊그제") return addDaysKst(today, -2);
-  if (s === "tomorrow" || s === "내일") return addDaysKst(today, 1);
-  if (s === "모레") return addDaysKst(today, 2);
+  if (s === "yesterday" || s === "어제") return addDays(today, -1);
+  if (s === "그저께" || s === "그제" || s === "엊그제") return addDays(today, -2);
+  if (s === "tomorrow" || s === "내일") return addDays(today, 1);
+  if (s === "모레") return addDays(today, 2);
   if (/^\d{4}-\d{2}-\d{2}$/.test(s)) return s as DateString;
   return null;
 }
@@ -113,7 +113,7 @@ export function resolveRequestedDate(
   if (typeof off === "number" && Number.isFinite(off)) {
     const n = Math.trunc(off);
     if (Math.abs(n) > MAX_DAY_OFFSET) return null;
-    return addDaysKst(today, n);
+    return addDays(today, n);
   }
   return hasDateStr ? null : today;
 }
diff --git a/src/services/judgmentService.ts b/src/services/judgmentService.ts
index ceca46a..daec012 100644
--- a/src/services/judgmentService.ts
+++ b/src/services/judgmentService.ts
@@ -15,7 +15,7 @@ import {
   thresholdsFor,
 } from "../constants/judgment";
 import type { DailyJudgment, RankSnapshot, VoteHistoryDoc } from "../types/panit";
-import { addDaysKst, tuesdayOf, type DateString } from "../types/dateString";
+import { addDays, tuesdayOf, type DateString } from "../types/dateString";
 
 /**
  * `(lastJudged, upTo)` 구간(양 끝 제외)에 **실제 판정 가능한 경기일**(=`thresholdsFor`가
@@ -34,12 +34,12 @@ export async function hasMissedGameDayBetween(
 ): Promise<boolean> {
   const fetch = gameCache ?? createGameDayCache();
   const MAX_LOOKBACK = 14;
-  let cursor = addDaysKst(upTo, -1);
+  let cursor = addDays(upTo, -1);
   for (let i = 0; i < MAX_LOOKBACK && cursor > lastJudged; i++) {
     const games = await fetch.listByDate(cursor);
     const completed = games.filter((g) => g.status === "completed").length;
     if (thresholdsFor(completed) !== "skip") return true;
-    cursor = addDaysKst(cursor, -1);
+    cursor = addDays(cursor, -1);
   }
   // MAX_LOOKBACK 초과로 종료된 경우엔 안전을 위해 결석으로 본다.
   return cursor > lastJudged;
@@ -84,7 +84,7 @@ export async function judgeDay(
   const lastJudgedPre = userPre?.lastJudgedDate;
   const streakBrokenIn =
     lastJudgedPre != null &&
-    lastJudgedPre < addDaysKst(date, -1) &&
+    lastJudgedPre < addDays(date, -1) &&
     (await hasMissedGameDayBetween(lastJudgedPre, date, fetch));
 
   const tx = await applyDailyJudgmentTx(uid, date, {
@@ -132,7 +132,7 @@ export async function judgeWeekIfNeeded(
 
   const byDate = new Map(entries.map((e) => [e.date, e.doc] as const));
   for (let i = 0; i < 6; i++) {
-    const d = addDaysKst(tuesday, i);
+    const d = addDays(tuesday, i);
     const doc = byDate.get(d);
     if (!doc || !doc.judgment) return; // 판정 누락
     if (doc.judgment === "fail") return;
diff --git a/src/services/statsService.ts b/src/services/statsService.ts
index 15fd9f8..3bafbac 100644
--- a/src/services/statsService.ts
+++ b/src/services/statsService.ts
@@ -7,7 +7,7 @@ import {getUser} from "../repositories/userRepository";
 import {hasMissedGameDayBetween} from "./judgmentService";
 import type {DailyJudgment, StatsResponse, VoteHistoryDoc} from "../types/panit";
 import {
-	addDaysKst,
+	addDays,
 	parseDateString,
 	toDateString,
 	startOfDayKst,
@@ -194,7 +194,7 @@ async function computeStats(uid: string, period: Period): Promise<StatsResponse>
 	//    KBO 월요일/올스타브레이크 등 휴장일은 자동 무시(`thresholdsFor === "skip"`).
 	// user doc은 갱신하지 않는다 — 다음 판정 시 `applyDailyJudgmentTx`가 정리한다.
 	const lastJudged = user?.lastJudgedDate;
-	const yesterday = addDaysKst(today, -1);
+	const yesterday = addDays(today, -1);
 	let streakBroken = false;
 	if (lastJudged != null && lastJudged < yesterday) {
 		const pending = await rtdb.ref(`/userVotes/${uid}/${yesterday}`).get();
diff --git a/src/types/dateString.ts b/src/types/dateString.ts
index f505e61..7147ab5 100644
--- a/src/types/dateString.ts
+++ b/src/types/dateString.ts
@@ -50,20 +50,20 @@ export function startOfDayKst(date: DateString): Date {
 
 /** 해당 KST 날짜가 속한 화~월 주의 화요일을 반환한다. */
 export function tuesdayOf(date: DateString): DateString {
-  const [y, m, d] = date.split("-").map(Number);
-  const dow = new Date(y, m - 1, d).getDay() || 7; // 일=7, 월=1, 화=2
+  // 요일 산정·날짜 가감 모두 TZ 독립 헬퍼로만 처리한다(로컬 Date 생성 금지).
+  const dow = dayOfWeek(date) || 7; // 일=7, 월=1, 화=2
   const offset = (dow - 2 + 7) % 7;
-  return toDateString(new Date(y, m - 1, d - offset));
+  return addDays(date, -offset);
 }
 
-/** `date`의 요일 (일=0, 월=1, ... 토=6). KST 기준. */
-export function dayOfWeekKst(date: DateString): number {
+/** `date`(달력 날짜)의 요일 (일=0, 월=1, … 토=6). 달력 날짜라 타임존 무관. */
+export function dayOfWeek(date: DateString): number {
   const [y, m, d] = date.split("-").map(Number);
   return new Date(y, m - 1, d).getDay();
 }
 
-/** `date`에 `delta` 일을 더한 KST 날짜를 반환. */
-export function addDaysKst(date: DateString, delta: number): DateString {
+/** `date`에 `delta` 일을 더한 날짜(YYYY-MM-DD). 순수 달력 산술이라 타임존 무관. */
+export function addDays(date: DateString, delta: number): DateString {
   const base = startOfDayKst(date).getTime();
   return toDateString(new Date(base + delta * 24 * 60 * 60 * 1000));
 }
diff --git a/tests/services/statsService.test.ts b/tests/services/statsService.test.ts
index 3dc9752..e4564c8 100644
--- a/tests/services/statsService.test.ts
+++ b/tests/services/statsService.test.ts
@@ -3,7 +3,7 @@ import { Timestamp } from "firebase-admin/firestore";
 import { firestore, rtdb } from "../../src/firebase";
 import { getStats } from "../../src/services/statsService";
 import {
-  addDaysKst,
+  addDays,
   todayKst,
   type DateString,
 } from "../../src/types/dateString";
@@ -70,8 +70,8 @@ describe("statsService.getStats — 결석 lazy 보정", () => {
 
   it("lastJudgedDate가 이틀 이상 이전이고 gap에 실제 경기일이 있으면 streak 0", async () => {
     const today = todayKst();
-    const threeDaysAgo = addDaysKst(today, -3);
-    const twoDaysAgo = addDaysKst(today, -2);
+    const threeDaysAgo = addDays(today, -3);
+    const twoDaysAgo = addDays(today, -2);
     await seedUser({
       currentStreak: 5,
       highestStreak: 7,
@@ -94,7 +94,7 @@ describe("statsService.getStats — 결석 lazy 보정", () => {
 
   it("gap에 실제 경기일이 없으면(전부 휴장) streak 유지", async () => {
     const today = todayKst();
-    const threeDaysAgo = addDaysKst(today, -3);
+    const threeDaysAgo = addDays(today, -3);
     await seedUser({
       currentStreak: 5,
       highestStreak: 7,
@@ -109,7 +109,7 @@ describe("statsService.getStats — 결석 lazy 보정", () => {
 
   it("lastJudgedDate가 어제(D-1)면 streak 유지", async () => {
     const today = todayKst();
-    const yesterday = addDaysKst(today, -1);
+    const yesterday = addDays(today, -1);
     await seedUser({
       currentStreak: 5,
       highestStreak: 5,
@@ -123,8 +123,8 @@ describe("statsService.getStats — 결석 lazy 보정", () => {
 
   it("lastJudgedDate가 D-2여도 어제분 userVotes가 남아 있으면 archive 대기로 보고 streak 보호", async () => {
     const today = todayKst();
-    const yesterday = addDaysKst(today, -1);
-    const twoDaysAgo = addDaysKst(today, -2);
+    const yesterday = addDays(today, -1);
+    const twoDaysAgo = addDays(today, -2);
     await seedUser({
       currentStreak: 5,
       highestStreak: 5,
@@ -142,8 +142,8 @@ describe("statsService.getStats — 결석 lazy 보정", () => {
 
   it("어제 경기가 있었는데 userVotes 비어 있으면(진짜 결석) streak 0", async () => {
     const today = todayKst();
-    const yesterday = addDaysKst(today, -1);
-    const twoDaysAgo = addDaysKst(today, -2);
+    const yesterday = addDays(today, -1);
+    const twoDaysAgo = addDays(today, -2);
     await seedUser({
       currentStreak: 5,
       highestStreak: 5,
@@ -207,7 +207,7 @@ describe("statsService.getStats — 캐시 forDate 검증", () => {
 
   it("캐시 forDate가 어제면 무효화하고 재계산한다", async () => {
     const today = todayKst();
-    const yesterday = addDaysKst(today, -1);
+    const yesterday = addDays(today, -1);
     await seedUser({ currentStreak: 3, lastJudgedDate: yesterday });
     // 어제 날짜의 stale 캐시를 심어둔다.
     await seedCache("current", {