feat(platform): checkpoint current implementation state

packages/engine/src/__tests__/duplicate-check.test.ts

@@ -96,4 +96,24 @@ describe("checkDuplicateAssignment", () => {
     const result = checkDuplicateAssignment("r1", "p1", "2026-04-01", "2026-05-01", [proposed]);
     expect(result.isDuplicate).toBe(true);
   });
+
+  it("formats conflict dates from local-midnight Date objects without shifting the calendar day", () => {
+    const localDateAssignment: ExistingAssignment = {
+      ...base,
+      startDate: new Date(2026, 3, 6),
+      endDate: new Date(2026, 3, 17),
+    };
+
+    const result = checkDuplicateAssignment(
+      "r1",
+      "p1",
+      new Date(2026, 3, 9),
+      new Date(2026, 3, 10),
+      [localDateAssignment],
+    );
+
+    expect(result.isDuplicate).toBe(true);
+    expect(result.message).toContain("2026-04-06");
+    expect(result.message).toContain("2026-04-17");
+  });
 });

packages/engine/src/allocation/duplicate-check.ts

@@ -20,10 +20,28 @@ export interface DuplicateCheckResult {
 
 const ACTIVE_STATUSES = new Set(["CONFIRMED", "ACTIVE", "PROPOSED"]);
 
-function toTime(d: Date | string): number {
-  const dt = typeof d === "string" ? new Date(d) : d;
-  dt.setHours(0, 0, 0, 0);
-  return dt.getTime();
+function toCalendarParts(value: Date | string): [year: number, month: number, day: number] {
+  if (typeof value === "string") {
+    const match = value.match(/^(\d{4})-(\d{2})-(\d{2})/u);
+    if (match) {
+      return [Number(match[1]), Number(match[2]), Number(match[3])];
+    }
+
+    const date = new Date(value);
+    return [date.getUTCFullYear(), date.getUTCMonth() + 1, date.getUTCDate()];
+  }
+
+  return [value.getFullYear(), value.getMonth() + 1, value.getDate()];
+}
+
+function toCalendarTime(value: Date | string): number {
+  const [year, month, day] = toCalendarParts(value);
+  return Date.UTC(year, month - 1, day);
+}
+
+function formatCalendarDate(value: Date | string): string {
+  const [year, month, day] = toCalendarParts(value);
+  return `${year}-${String(month).padStart(2, "0")}-${String(day).padStart(2, "0")}`;
 }
 
 /**
@@ -45,8 +63,8 @@ export function checkDuplicateAssignment(
   existingAssignments: ExistingAssignment[],
   excludeAssignmentId?: string,
 ): DuplicateCheckResult {
-  const newStart = toTime(startDate);
-  const newEnd = toTime(endDate);
+  const newStart = toCalendarTime(startDate);
+  const newEnd = toCalendarTime(endDate);
 
   for (const existing of existingAssignments) {
     // Skip self (for updates)
@@ -60,12 +78,12 @@ export function checkDuplicateAssignment(
     if (!ACTIVE_STATUSES.has(existing.status)) continue;
 
     // Check date overlap: existingStart <= newEnd && existingEnd >= newStart
-    const existStart = toTime(existing.startDate);
-    const existEnd = toTime(existing.endDate);
+    const existStart = toCalendarTime(existing.startDate);
+    const existEnd = toCalendarTime(existing.endDate);
 
     if (existStart <= newEnd && existEnd >= newStart) {
-      const startStr = new Date(existing.startDate).toISOString().slice(0, 10);
-      const endStr = new Date(existing.endDate).toISOString().slice(0, 10);
+      const startStr = formatCalendarDate(existing.startDate);
+      const endStr = formatCalendarDate(existing.endDate);
       return {
         isDuplicate: true,
         conflictingAssignment: existing,