feat: add birth date value adapter

frontend/package-lock.json

@@ -15,6 +15,7 @@
         "@tailwindcss/postcss": "^4.3.2",
         "class-variance-authority": "^0.7.1",
         "clsx": "^2.1.1",
+        "date-fns": "^4.4.0",
         "lucide-react": "^1.24.0",
         "next": "16.2.10",
         "react": "19.2.4",
@@ -4131,6 +4132,16 @@
         "url": "https://github.com/sponsors/ljharb"
       }
     },
+    "node_modules/date-fns": {
+      "version": "4.4.0",
+      "resolved": "https://registry.npmjs.org/date-fns/-/date-fns-4.4.0.tgz",
+      "integrity": "sha512-+1UMbeh68lH1SegH83CGWwpb6OHHbpSgr3+s5Eww5M4CAgswBpoWS0AjTOfEJ33HiYKz1hdj/KTFprzXHmq/6w==",
+      "license": "MIT",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/kossnocorp"
+      }
+    },
     "node_modules/debug": {
       "version": "4.4.3",
       "resolved": "https://registry.npmjs.org/debug/-/debug-4.4.3.tgz",

frontend/package.json

@@ -19,6 +19,7 @@
     "@tailwindcss/postcss": "^4.3.2",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
+    "date-fns": "^4.4.0",
     "lucide-react": "^1.24.0",
     "next": "16.2.10",
     "react": "19.2.4",

frontend/src/lib/birth-time-intake-model.ts

@@ -1,5 +1,8 @@
+import { format, isValid, parse } from "date-fns";
 import type { JourneySnapshot } from "./birth-time-journey.ts";
 
+const birthDatePattern = "yyyy-MM-dd";
+
 export type BirthTimeSource =
   | ""
   | "hospital_record"
@@ -39,6 +42,17 @@ export type BirthTimeDraft = {
 
 export type BirthTimeDraftPatch = Partial<BirthTimeDraft>;
 
+export function parseBirthDate(value: string): Date | undefined {
+  if (value === "") return undefined;
+  const parsed = parse(value, birthDatePattern, new Date(2000, 0, 1));
+  if (!isValid(parsed) || format(parsed, birthDatePattern) !== value) return undefined;
+  return parsed;
+}
+
+export function formatBirthDate(value: Date): string {
+  return format(value, birthDatePattern);
+}
+
 export const birthTimeSourceOptions = [
   { value: "hospital_record", label: "出生证明或医院记录", hint: "先检查前后两分钟是否稳定" },
   { value: "family_exact", label: "家人明确记得具体时间", hint: "进行 5—15 分钟轻量校正" },

frontend/tests/birth-time-intake.test.ts

@@ -4,7 +4,9 @@ import {
   assistantIntentCopy,
   birthTimePersistenceValues,
   describeBirthTimeDraft,
+  formatBirthDate,
   isBirthTimeDraftReady,
+  parseBirthDate,
   type BirthTimeDraft,
 } from "../src/lib/birth-time-intake-model.ts";
 
@@ -79,3 +81,19 @@ test("birth time intake describes uncertainty without claiming false precision",
     "目前只能保存候选范围，还没有足够证据应用到具体分钟。",
   );
 });
+
+test("birth date parsing preserves the local calendar day", () => {
+  const parsed = parseBirthDate("1993-04-17");
+
+  assert.equal(parsed?.getFullYear(), 1993);
+  assert.equal(parsed?.getMonth(), 3);
+  assert.equal(parsed?.getDate(), 17);
+});
+
+test("birth date values round trip leap days and reject invalid input", () => {
+  const leapDay = parseBirthDate("2000-02-29");
+
+  assert.equal(leapDay === undefined ? undefined : formatBirthDate(leapDay), "2000-02-29");
+  assert.equal(parseBirthDate(""), undefined);
+  assert.equal(parseBirthDate("2001-02-29"), undefined);
+});