diff --git a/lib/calculate.ts b/lib/calculate.ts
index dc0d905b0..8d8ce7005 100644
--- a/lib/calculate.ts
+++ b/lib/calculate.ts
@@ -557,8 +557,16 @@ export function chunkDaysIntoWeeks(
     if (!day.date || typeof day.date !== 'string' || day.date.trim() === '') return false;
     // Validate YYYY-MM-DD format
     if (!/^\d{4}-\d{2}-\d{2}$/.test(day.date)) return false;
-    // Ensure it's a valid date
-    const dateObj = new Date(day.date);
+    // Validate date components (reject impossible dates like 2024-02-30)
+    const parts = day.date.match(/^(\d{4})-(\d{2})-(\d{2})$/);
+    if (!parts) return false;
+    const year = parseInt(parts[1], 10);
+    const month = parseInt(parts[2], 10);
+    const dayNum = parseInt(parts[3], 10);
+    if (month < 1 || month > 12) return false;
+    const daysInMonth = new Date(year, month, 0).getDate();
+    if (dayNum < 1 || dayNum > daysInMonth) return false;
+    const dateObj = new Date(year, month - 1, dayNum);
     return !isNaN(dateObj.getTime());
   });