Reject impossible LCK calendar dates

Date-driven LCK lookups should fail fast for impossible user input instead of silently returning an empty match list. The parser now validates month/day bounds with explicit leap-year handling, and the tests lock both direct normalization and schedule normalization behavior.

Constraint: PR #55 review requested a regression for 2026-02-31 before implementation
Rejected: Rely on Date parsing round-trip | explicit bounds avoid timezone and overflow normalization surprises
Confidence: high
Scope-risk: narrow
Directive: Keep date validation before schedule filtering so invalid user dates cannot become misleading no-match responses
Tested: node --test packages/lck-analytics/test/index.test.js scripts/skill-docs.test.js
Tested: npm run lint --workspace lck-analytics
Tested: npm test --workspace lck-analytics
Tested: npm run ci
Tested: getMatchResults('2026-02-31') rejection smoke
Tested: getLckSummary('2026-04-01', { team: '한화', includeStandings: true }) live smoke
Tested: lck-analytics script smokes for sync-oracle, build-match-report, and analyze-live-game

packages/lck-analytics/src/parse.js

@@ -38,8 +38,7 @@ function normalizeDateInput(value) {
   }
 
   const [year, month, day] = [match[1], match[2], match[3]];
-  const candidate = new Date(`${year}-${month}-${day}T00:00:00+09:00`);
-  if (Number.isNaN(candidate.getTime())) {
+  if (!isValidCalendarDate(year, month, day)) {
     throw new Error("date must be a valid Date or YYYY-MM-DD string.");
   }
 
@@ -51,6 +50,40 @@ function normalizeDateInput(value) {
   };
 }
 
+function isValidCalendarDate(year, month, day) {
+  const numericYear = Number(year);
+  const numericMonth = Number(month);
+  const numericDay = Number(day);
+
+  if (!Number.isInteger(numericYear) || !Number.isInteger(numericMonth) || !Number.isInteger(numericDay)) {
+    return false;
+  }
+
+  if (numericMonth < 1 || numericMonth > 12 || numericDay < 1) {
+    return false;
+  }
+
+  const daysInMonth = [
+    31,
+    isLeapYear(numericYear) ? 29 : 28,
+    31,
+    30,
+    31,
+    30,
+    31,
+    31,
+    30,
+    31,
+    30,
+    31,
+  ][numericMonth - 1];
+  return numericDay <= daysInMonth;
+}
+
+function isLeapYear(year) {
+  return year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0);
+}
+
 function eventToKoreaDateTime(startTime) {
   const date = new Date(startTime);
   const parts = new Intl.DateTimeFormat("en-CA", {

packages/lck-analytics/test/index.test.js

@@ -13,6 +13,7 @@ const {
   parseOracleCsv,
 } = require("../src/index");
 const {
+  normalizeDateInput,
   normalizeScheduleResponse,
   normalizeStandingsResponse,
 } = require("../src/parse");
@@ -45,6 +46,22 @@ test("normalizeScheduleResponse filters requested LCK date and Korean team alias
   assert.deepEqual(result.matches[0].score, { team1: 1, team2: 0 });
 });
 
+test("date normalization rejects impossible calendar dates", () => {
+  assert.equal(normalizeDateInput("2024-02-29").isoDate, "2024-02-29");
+  assert.throws(
+    () => normalizeDateInput("2026-02-31"),
+    /date must be a valid Date or YYYY-MM-DD string\./,
+  );
+  assert.throws(
+    () => normalizeDateInput("2026-02-29"),
+    /date must be a valid Date or YYYY-MM-DD string\./,
+  );
+  assert.throws(
+    () => normalizeScheduleResponse(schedulePayload, { date: "2026-02-31" }),
+    /date must be a valid Date or YYYY-MM-DD string\./,
+  );
+});
+
 test("normalizeStandingsResponse keeps the LCK standings shape and alias resolution", () => {
   const table = normalizeStandingsResponse(standingsPayload, {
     tournament: {