Apply linen design system across app

src/lib/child-birthdate.ts

@@ -0,0 +1,34 @@
+export const CHILD_MAX_AGE_YEARS = 7;
+
+export function formatDateInputValue(date: Date) {
+  const year = date.getFullYear();
+  const month = String(date.getMonth() + 1).padStart(2, "0");
+  const day = String(date.getDate()).padStart(2, "0");
+
+  return `${year}-${month}-${day}`;
+}
+
+export function getChildBirthdateRange(referenceDate = new Date()) {
+  const max = new Date(
+    referenceDate.getFullYear(),
+    referenceDate.getMonth(),
+    referenceDate.getDate(),
+  );
+  const min = new Date(max);
+  min.setFullYear(min.getFullYear() - CHILD_MAX_AGE_YEARS);
+
+  return {
+    min: formatDateInputValue(min),
+    max: formatDateInputValue(max),
+  };
+}
+
+export function parseDateInput(value?: string) {
+  if (!value) return null;
+  if (!/^\d{4}-\d{2}-\d{2}$/.test(value)) return null;
+
+  const date = new Date(`${value}T00:00:00`);
+  if (Number.isNaN(date.getTime())) return null;
+
+  return formatDateInputValue(date) === value ? date : null;
+}