diff --git a/app/modules/cupang/db.py b/app/modules/cupang/db.py
index 0d2abf3..17ab9f0 100644
--- a/app/modules/cupang/db.py
+++ b/app/modules/cupang/db.py
@@ -20,6 +20,8 @@ from typing import Any
 from psycopg.rows import dict_row
 from psycopg_pool import ConnectionPool
 
+from app.timezone import KST
+
 from .store import STATUSES, compute_boxes
 
 
@@ -558,7 +560,7 @@ class CupangDBStore:
         for k in ("created_at", "updated_at"):
             v = out.get(k)
             if isinstance(v, datetime):
-                out[k] = v.astimezone(timezone.utc).isoformat(timespec="seconds")
+                out[k] = v.astimezone(KST).isoformat(timespec="seconds")
         return out
 
     @staticmethod
@@ -572,7 +574,7 @@ class CupangDBStore:
         for k in ("created_at", "updated_at"):
             v = out.get(k)
             if isinstance(v, datetime):
-                out[k] = v.astimezone(timezone.utc).isoformat(timespec="seconds")
+                out[k] = v.astimezone(KST).isoformat(timespec="seconds")
         return out
 
     @staticmethod
@@ -586,7 +588,7 @@ class CupangDBStore:
         for k in ("created_at", "updated_at"):
             v = out.get(k)
             if isinstance(v, datetime):
-                out[k] = v.astimezone(timezone.utc).isoformat(timespec="seconds")
+                out[k] = v.astimezone(KST).isoformat(timespec="seconds")
         return out
 
     @staticmethod
@@ -603,7 +605,7 @@ class CupangDBStore:
         for k in ("created_at", "updated_at"):
             v = out.get(k)
             if isinstance(v, datetime):
-                out[k] = v.astimezone(timezone.utc).isoformat(timespec="seconds")
+                out[k] = v.astimezone(KST).isoformat(timespec="seconds")
         return out
 
     @staticmethod
@@ -620,5 +622,5 @@ class CupangDBStore:
         for k in ("created_at", "updated_at"):
             v = out.get(k)
             if isinstance(v, datetime):
-                out[k] = v.astimezone(timezone.utc).isoformat(timespec="seconds")
+                out[k] = v.astimezone(KST).isoformat(timespec="seconds")
         return out
diff --git a/app/modules/cupang/router.py b/app/modules/cupang/router.py
index b1b4137..d97b85b 100644
--- a/app/modules/cupang/router.py
+++ b/app/modules/cupang/router.py
@@ -11,9 +11,10 @@ from __future__ import annotations
 
 import calendar as _calendar
 import json
-from datetime import date
 from typing import Any
 
+from app.timezone import today_kst
+
 from fastapi import APIRouter, Body, Depends, Form, HTTPException, Request
 from fastapi.responses import HTMLResponse, JSONResponse, RedirectResponse
 
@@ -98,7 +99,7 @@ def _parse_lines(lines_json: str) -> list[dict[str, Any]]:
 
 
 def _ym(request: Request) -> tuple[int, int]:
-    today = date.today()
+    today = today_kst()
     try:
         year = int(request.query_params.get("year") or today.year)
         month = int(request.query_params.get("month") or today.month)
@@ -128,7 +129,7 @@ async def index(request: Request) -> HTMLResponse:
 
     # 선택 날짜 (기본: 오늘이 이번 달이면 오늘, 아니면 1일)
     sel = request.query_params.get("date") or ""
-    today = date.today()
+    today = today_kst()
     if not sel:
         sel = today.isoformat() if (today.year == year and today.month == month) else f"{year:04d}-{month:02d}-01"
 
@@ -225,7 +226,7 @@ async def new_form(request: Request) -> HTMLResponse:
             "page_subtitle": "공통 헤더 1개 + 품목 라인",
             "mode": "new",
             "shipment": None,
-            "default_date": date.today().isoformat(),
+            "default_date": today_kst().isoformat(),
         }
     )
     return render_template(request, "cupang/form.html", ctx)
diff --git a/app/modules/expense/db.py b/app/modules/expense/db.py
index da9896a..1cace89 100644
--- a/app/modules/expense/db.py
+++ b/app/modules/expense/db.py
@@ -18,6 +18,8 @@ from typing import Any
 from psycopg.rows import dict_row
 from psycopg_pool import ConnectionPool
 
+from app.timezone import KST
+
 from .store import CATEGORIES, METHODS, STATUSES
 
 
@@ -364,7 +366,7 @@ class ExpenseDBStore:
         out = dict(row)
         v = out.get("uploaded_at")
         if isinstance(v, datetime):
-            out["uploaded_at"] = v.astimezone(timezone.utc).isoformat(timespec="seconds")
+            out["uploaded_at"] = v.astimezone(KST).isoformat(timespec="seconds")
         out["size_bytes"] = int(out.get("size_bytes", 0))
         return out
 
@@ -468,7 +470,7 @@ class ExpenseDBStore:
         for k in ("created_at", "updated_at", "decided_at"):
             v = out.get(k)
             if isinstance(v, datetime):
-                out[k] = v.astimezone(timezone.utc).isoformat(timespec="seconds")
+                out[k] = v.astimezone(KST).isoformat(timespec="seconds")
         out["amount"] = int(out.get("amount", 0))
         return out
 
diff --git a/app/modules/expense/router.py b/app/modules/expense/router.py
index 70354ed..2f879ce 100644
--- a/app/modules/expense/router.py
+++ b/app/modules/expense/router.py
@@ -13,10 +13,11 @@ import mimetypes
 import os
 import re
 import uuid
-from datetime import datetime
 from pathlib import Path
 from typing import Any
 
+from app.timezone import now_kst
+
 from fastapi import (
     APIRouter,
     Depends,
@@ -231,11 +232,11 @@ async def reports_page(request: Request) -> HTMLResponse:
         return RedirectResponse(url="/login", status_code=303)
     store = _store(request)
     _require_db_store(store)
-    year_str = request.query_params.get("year") or str(datetime.now().year)
+    year_str = request.query_params.get("year") or str(now_kst().year)
     try:
         year = int(year_str)
     except ValueError:
-        year = datetime.now().year
+        year = now_kst().year
     rows = store.monthly_summary(email=user["email"], year=year)
 
     # 피벗: month → {category → total}
@@ -633,7 +634,7 @@ async def export_xlsx(
     buf.seek(0)
 
     fname_scope = "all" if target_email is None else "mine"
-    fname = f"expense_{fname_scope}_{datetime.now().strftime('%Y%m%d_%H%M%S')}.xlsx"
+    fname = f"expense_{fname_scope}_{now_kst().strftime('%Y%m%d_%H%M%S')}.xlsx"
     return StreamingResponse(
         buf,
         media_type="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
diff --git a/app/modules/expense/store.py b/app/modules/expense/store.py
index cd7e96e..3317c43 100644
--- a/app/modules/expense/store.py
+++ b/app/modules/expense/store.py
@@ -13,17 +13,18 @@ import os
 import tempfile
 import threading
 import uuid
-from datetime import datetime, timezone
 from pathlib import Path
 from typing import Any
 
+from app.timezone import now_kst_iso
+
 CATEGORIES: tuple[str, ...] = ("식대", "교통", "숙박", "비품", "접대", "통신", "기타")
 METHODS: tuple[str, ...] = ("법인카드", "개인지출", "현금")
 STATUSES: tuple[str, ...] = ("작성중", "제출", "승인", "반려", "정산완료")
 
 
 def _now_iso() -> str:
-    return datetime.now(timezone.utc).isoformat(timespec="seconds")
+    return now_kst_iso()
 
 
 class ExpenseStore:
diff --git a/app/modules/vacation/db.py b/app/modules/vacation/db.py
index deed4ee..b2c92b8 100644
--- a/app/modules/vacation/db.py
+++ b/app/modules/vacation/db.py
@@ -22,6 +22,8 @@ from typing import Any
 from psycopg.rows import dict_row
 from psycopg_pool import ConnectionPool
 
+from app.timezone import KST
+
 from .store import EDITABLE_STATUSES, STATUSES, VACATION_TYPES, compute_days, normalize_half
 
 
@@ -449,7 +451,7 @@ class VacationDBStore:
         for k in ("created_at", "updated_at", "decided_at"):
             v = out.get(k)
             if isinstance(v, datetime):
-                out[k] = v.astimezone(timezone.utc).isoformat(timespec="seconds")
+                out[k] = v.astimezone(KST).isoformat(timespec="seconds")
         out["days"] = cls._num(out.get("days", 0))
         return out
 
@@ -466,7 +468,7 @@ class VacationDBStore:
         for k in ("created_at", "updated_at"):
             v = out.get(k)
             if isinstance(v, datetime):
-                out[k] = v.astimezone(timezone.utc).isoformat(timespec="seconds")
+                out[k] = v.astimezone(KST).isoformat(timespec="seconds")
         return out
 
     @classmethod
@@ -481,5 +483,5 @@ class VacationDBStore:
         for k in ("created_at", "updated_at"):
             v = out.get(k)
             if isinstance(v, datetime):
-                out[k] = v.astimezone(timezone.utc).isoformat(timespec="seconds")
+                out[k] = v.astimezone(KST).isoformat(timespec="seconds")
         return out
diff --git a/app/modules/vacation/router.py b/app/modules/vacation/router.py
index dc18896..a4601a3 100644
--- a/app/modules/vacation/router.py
+++ b/app/modules/vacation/router.py
@@ -11,7 +11,7 @@ from __future__ import annotations
 
 import calendar as _calendar
 import io
-from datetime import date, datetime
+from datetime import date
 from typing import Any
 
 from fastapi import APIRouter, Depends, Form, HTTPException, Request
@@ -22,6 +22,8 @@ from fastapi.responses import (
     StreamingResponse,
 )
 
+from app.timezone import now_kst, today_kst
+
 from .store import HALF_LABELS, HALVES, VACATION_TYPES
 
 router = APIRouter(prefix="/vacation", tags=["vacation"])
@@ -107,7 +109,7 @@ def _guard(
 
 
 def _ym(request: Request) -> tuple[int, int]:
-    today = date.today()
+    today = today_kst()
     try:
         year = int(request.query_params.get("year") or today.year)
         month = int(request.query_params.get("month") or today.month)
@@ -170,7 +172,7 @@ def _build_calendar(
         date_from=range_start.isoformat(), date_to=range_end.isoformat()
     )
 
-    today = date.today()
+    today = today_kst()
     weeks: list[dict[str, Any]] = []
     for week in weeks_dates:
         w_start, w_end = week[0], week[-1]
@@ -234,7 +236,7 @@ async def index(request: Request) -> HTMLResponse:
 
     year, month = _ym(request)
     sel = request.query_params.get("date") or ""
-    today = date.today()
+    today = today_kst()
     if not sel:
         sel = (
             today.isoformat()
@@ -315,7 +317,7 @@ async def new_form(request: Request) -> HTMLResponse:
             "page_subtitle": "휴가 종류 · 기간 · 사유 입력",
             "mode": "new",
             "req": None,
-            "default_date": date.today().isoformat(),
+            "default_date": today_kst().isoformat(),
         }
     )
     return render_template(request, "vacation/form.html", ctx)
@@ -419,7 +421,7 @@ async def settings_page(request: Request) -> HTMLResponse:
             {"reason": "휴가 설정은 관리자만 접근할 수 있습니다.", "is_admin": False},
             status_code=403,
         )
-    today = date.today()
+    today = today_kst()
     try:
         year = int(request.query_params.get("year") or today.year)
     except ValueError:
@@ -551,7 +553,7 @@ async def export_xlsx(
     if store is None:
         raise HTTPException(status_code=503, detail="vacation_db 미설정")
 
-    today = date.today()
+    today = today_kst()
     try:
         year = int(request.query_params.get("year") or today.year)
         month = int(request.query_params.get("month") or 0)
@@ -599,7 +601,7 @@ async def export_xlsx(
     wb.save(buf)
     buf.seek(0)
     scope = f"{year}{('%02d' % month) if (1 <= month <= 12) else ''}"
-    fname = f"vacation_{scope}_{datetime.now().strftime('%Y%m%d_%H%M%S')}.xlsx"
+    fname = f"vacation_{scope}_{now_kst().strftime('%Y%m%d_%H%M%S')}.xlsx"
     return StreamingResponse(
         buf,
         media_type="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
diff --git a/app/store.py b/app/store.py
index 8b3f2ae..1612b1f 100644
--- a/app/store.py
+++ b/app/store.py
@@ -10,10 +10,11 @@ import json
 import os
 import tempfile
 import threading
-from datetime import datetime, timezone
 from pathlib import Path
 from typing import Any, Iterable
 
+from .timezone import now_kst_iso
+
 # 슈퍼 관리자 — 강등/삭제 불가
 SUPER_ADMIN_EMAIL = "king@dbxcorp.co.kr"
 
@@ -36,7 +37,7 @@ APPROVER_KEYS: tuple[str, ...] = ("expense_approver", "vacation_approver")
 
 
 def _now_iso() -> str:
-    return datetime.now(timezone.utc).isoformat(timespec="seconds")
+    return now_kst_iso()
 
 
 class UserStore:
diff --git a/app/timezone.py b/app/timezone.py
new file mode 100644
index 0000000..54c25fa
--- /dev/null
+++ b/app/timezone.py
@@ -0,0 +1,38 @@
+"""프로젝트 공통 시간대 — 모든 시간은 한국 시간(KST, UTC+9)으로 표시/계산한다.
+
+대한민국은 서머타임(DST)이 없으므로 고정 오프셋 +9 로 충분하다.
+zoneinfo/tzdata 의존 없이 어디서나 동일하게 동작한다.
+
+- DB 의 TIMESTAMPTZ 는 UTC 로 저장되고 psycopg 가 aware datetime(UTC)로 돌려준다.
+  표시 직전에 `to_kst_iso()` 로 KST 문자열로 변환한다.
+- "오늘"/"지금" 판정은 `today_kst()` / `now_kst()` 를 쓴다(서버 로컬 TZ 무관).
+"""
+
+from __future__ import annotations
+
+from datetime import date, datetime, timedelta, timezone
+
+KST = timezone(timedelta(hours=9), name="KST")
+
+
+def now_kst() -> datetime:
+    """현재 시각(KST, tz-aware)."""
+    return datetime.now(KST)
+
+
+def today_kst() -> date:
+    """오늘 날짜(KST 기준)."""
+    return now_kst().date()
+
+
+def now_kst_iso() -> str:
+    return now_kst().isoformat(timespec="seconds")
+
+
+def to_kst_iso(dt: datetime | None, *, timespec: str = "seconds") -> str | None:
+    """datetime → KST ISO 문자열. naive 는 UTC 로 간주 후 변환."""
+    if dt is None:
+        return None
+    if dt.tzinfo is None:
+        dt = dt.replace(tzinfo=timezone.utc)
+    return dt.astimezone(KST).isoformat(timespec=timespec)