diff --git a/apps/web/app/onboarding/(questions)/age/page.tsx b/apps/web/app/onboarding/(questions)/age/page.tsx
index 89b7f28..17adca6 100644
--- a/apps/web/app/onboarding/(questions)/age/page.tsx
+++ b/apps/web/app/onboarding/(questions)/age/page.tsx
@@ -6,37 +6,12 @@ import { useRouter } from "next/navigation";
 import { type FormEvent, useEffect, useRef } from "react";
 import { Controller, useFormContext, useWatch } from "react-hook-form";
 import { useSaveOnboardingProfile } from "@/app/onboarding/_hooks/use-save-onboarding-profile";
-import { onlyDigits } from "@/lib/number";
-
-function formatBirthDateInput(value: string) {
-  const digits = onlyDigits(value).slice(0, 8);
-  return [digits.slice(0, 4), digits.slice(4, 6), digits.slice(6, 8)].filter(Boolean).join(".");
-}
-
-function toBirthDate(value: string) {
-  return /^\d{4}\.\d{2}\.\d{2}$/.test(value) ? value.replaceAll(".", "-") : value;
-}
-
-function isRealBirthDate(value: string) {
-  if (!/^\d{4}-\d{2}-\d{2}$/.test(value)) return false;
-
-  const parts = value.split("-");
-  const year = Number(parts[0]);
-  const month = Number(parts[1]);
-  const day = Number(parts[2]);
-  const date = new Date(Date.UTC(year, month - 1, day));
-  return (
-    date.getUTCFullYear() === year && date.getUTCMonth() === month - 1 && date.getUTCDate() === day
-  );
-}
-
-/** 서버는 생년월일을 과거 날짜로만 받는다(오늘도 거부) — 보내기 전에 같은 기준으로 거른다. */
-function isPastBirthDate(value: string) {
-  const today = new Date();
-  const todayUtc = Date.UTC(today.getFullYear(), today.getMonth(), today.getDate());
-  const parts = value.split("-");
-  return Date.UTC(Number(parts[0]), Number(parts[1]) - 1, Number(parts[2])) < todayUtc;
-}
+import {
+  formatBirthDateInput,
+  isPastBirthDate,
+  isRealBirthDate,
+  toIsoBirthDate,
+} from "@/lib/birth-date";
 
 export default function AgeOnboardingPage() {
   const router = useRouter();
@@ -107,7 +82,7 @@ export default function AgeOnboardingPage() {
                 placeholder="YYYY.MM.DD"
                 value={field.value.replaceAll("-", ".")}
                 onChange={(event) =>
-                  field.onChange(toBirthDate(formatBirthDateInput(event.target.value)))
+                  field.onChange(toIsoBirthDate(formatBirthDateInput(event.target.value)))
                 }
                 onKeyDown={(event) => {
                   // 한글 등 IME 조합을 확정하는 Enter는 다음 단계로 넘기지 않는다.
diff --git a/apps/web/app/profile/edit/page.tsx b/apps/web/app/profile/edit/page.tsx
index b36318f..9d9958b 100644
--- a/apps/web/app/profile/edit/page.tsx
+++ b/apps/web/app/profile/edit/page.tsx
@@ -7,6 +7,12 @@ import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { ChevronLeft } from "lucide-react";
 import { useRouter, useSearchParams } from "next/navigation";
 import { type FormEvent, useEffect, useState } from "react";
+import {
+  formatBirthDateInput,
+  isPastBirthDate,
+  isRealBirthDate,
+  toIsoBirthDate,
+} from "@/lib/birth-date";
 import { SAVE_FAILED_TEXT } from "@/lib/messages";
 import { onlyDigits } from "@/lib/number";
 import { updateGoalOptions } from "@/lib/queries/goal";
@@ -33,29 +39,6 @@ const PERIOD_OPTIONS = [
   { label: "3년 미만", months: 36 },
 ] as const;
 
-function formatBirthDateInput(value: string) {
-  const digits = onlyDigits(value).slice(0, 8);
-  return [digits.slice(0, 4), digits.slice(4, 6), digits.slice(6, 8)].filter(Boolean).join(".");
-}
-
-function toIsoBirthDate(value: string) {
-  return /^\d{4}\.\d{2}\.\d{2}$/.test(value) ? value.replaceAll(".", "-") : value;
-}
-
-function isValidPastDate(value: string) {
-  if (!/^\d{4}-\d{2}-\d{2}$/.test(value)) return false;
-
-  const [year, month, day] = value.split("-").map(Number);
-  if (year === undefined || month === undefined || day === undefined) return false;
-
-  const date = new Date(Date.UTC(year, month - 1, day));
-  const isRealDate =
-    date.getUTCFullYear() === year && date.getUTCMonth() === month - 1 && date.getUTCDate() === day;
-  const today = new Date();
-  const todayUtc = Date.UTC(today.getFullYear(), today.getMonth(), today.getDate());
-  return isRealDate && date.getTime() < todayUtc;
-}
-
 function selectedPeriodOption(periodMonths: number) {
   if (periodMonths <= 12) return 12;
   if (periodMonths <= 24) return 24;
@@ -160,7 +143,7 @@ export default function ProfileEditPage() {
     const monthlySavingManwon = Number(draft.monthlySavingManwon);
     const netWorthManwon = Number(draft.netWorthManwon);
 
-    if (!isValidPastDate(birthDate)) {
+    if (!isRealBirthDate(birthDate) || !isPastBirthDate(birthDate)) {
       setSubmitError("올바른 생년월일을 입력해 주세요.");
       return;
     }
diff --git a/apps/web/lib/birth-date.test.ts b/apps/web/lib/birth-date.test.ts
new file mode 100644
index 0000000..2dd7b17
--- /dev/null
+++ b/apps/web/lib/birth-date.test.ts
@@ -0,0 +1,42 @@
+import { describe, expect, it, vi } from "vitest";
+import {
+  formatBirthDateInput,
+  isPastBirthDate,
+  isRealBirthDate,
+  toIsoBirthDate,
+} from "./birth-date";
+
+describe("birth date utilities", () => {
+  it("부분 입력을 점 구분 표기로 정규화한다", () => {
+    expect(formatBirthDateInput("1998")).toBe("1998");
+    expect(formatBirthDateInput("199803")).toBe("1998.03");
+    expect(formatBirthDateInput("1998.03.01abc")).toBe("1998.03.01");
+  });
+
+  it("완성된 점 구분 표기만 ISO 날짜로 바꾼다", () => {
+    expect(toIsoBirthDate("2002.10.24")).toBe("2002-10-24");
+    expect(toIsoBirthDate("2002.10")).toBe("2002.10");
+    expect(toIsoBirthDate("2002-10-24")).toBe("2002-10-24");
+  });
+
+  it("실제로 존재하는 날짜인지 검증한다", () => {
+    expect(isRealBirthDate("2000-02-29")).toBe(true);
+    expect(isRealBirthDate("1900-02-29")).toBe(false);
+    expect(isRealBirthDate("2002-10")).toBe(false);
+  });
+
+  it("오늘 이전의 실재하는 날짜만 과거 날짜로 인정한다", () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date(2026, 8, 11, 12));
+
+    try {
+      expect(isPastBirthDate("2026-09-10")).toBe(true);
+      expect(isPastBirthDate("2026-09-11")).toBe(false);
+      expect(isPastBirthDate("2026-09-12")).toBe(false);
+      expect(isPastBirthDate("1900-02-29")).toBe(false);
+      expect(isPastBirthDate("2002-10")).toBe(false);
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+});
diff --git a/apps/web/lib/birth-date.ts b/apps/web/lib/birth-date.ts
new file mode 100644
index 0000000..eb483b3
--- /dev/null
+++ b/apps/web/lib/birth-date.ts
@@ -0,0 +1,33 @@
+import { onlyDigits } from "./number";
+
+export function formatBirthDateInput(value: string) {
+  const digits = onlyDigits(value).slice(0, 8);
+  return [digits.slice(0, 4), digits.slice(4, 6), digits.slice(6, 8)].filter(Boolean).join(".");
+}
+
+export function toIsoBirthDate(value: string) {
+  return /^\d{4}\.\d{2}\.\d{2}$/.test(value) ? value.replaceAll(".", "-") : value;
+}
+
+export function isRealBirthDate(value: string) {
+  if (!/^\d{4}-\d{2}-\d{2}$/.test(value)) return false;
+
+  const parts = value.split("-");
+  const year = Number(parts[0]);
+  const month = Number(parts[1]);
+  const day = Number(parts[2]);
+  const date = new Date(Date.UTC(year, month - 1, day));
+  return (
+    date.getUTCFullYear() === year && date.getUTCMonth() === month - 1 && date.getUTCDate() === day
+  );
+}
+
+/** 서버는 생년월일을 과거 날짜로만 받는다(오늘도 거부) — 보내기 전에 같은 기준으로 거른다. */
+export function isPastBirthDate(value: string) {
+  if (!isRealBirthDate(value)) return false;
+
+  const parts = value.split("-");
+  const today = new Date();
+  const todayUtc = Date.UTC(today.getFullYear(), today.getMonth(), today.getDate());
+  return Date.UTC(Number(parts[0]), Number(parts[1]) - 1, Number(parts[2])) < todayUtc;
+}