@codecademy/gamut

Index: package/dist/DatePicker/Calendar/utils/dateGrid.js
===================================================================
--- package/dist/DatePicker/Calendar/utils/dateGrid.js
+++ package/dist/DatePicker/Calendar/utils/dateGrid.js
@@ -0,0 +1,103 @@
+/**
+ * Builds a grid of days for a calendar month using native Date and Intl.
+ * Each row has 7 cells; leading/trailing cells may be null (padding from adjacent months).
+ */
+
+const DAYS_PER_WEEK = 7;
+
+/**
+ * Normalize to start of day in local time for comparison.
+ */
+const normalizeDate = date => {
+  return new Date(date.getFullYear(), date.getMonth(), date.getDate()).getTime();
+};
+
+/**
+ * Number of empty cells before the 1st of the month, for a grid whose first column is
+ * `firstWeekday` (ISO: 1 = Monday … 7 = Sunday from `Intl.Locale#getWeekInfo`).
+ */
+export const getWeekdayOffsetInGrid = (date, firstWeekday) => {
+  const js = date.getDay();
+  const iso = js === 0 ? 7 : js;
+  return (iso - firstWeekday + 14) % 7;
+};
+
+/**
+ * Returns an array of weeks for the given month. Each week is an array of 7 items:
+ * each item is either a Date (that day) or null (padding from previous/next month).
+ *
+ * @param year - Full year (e.g. 2026)
+ * @param month - Month 0-11 (0 = January)
+ * @param firstWeekday - First day of the week for the calendar row (ISO 1–7, from `getWeekInfo().firstDay`)
+ */
+export const getMonthGrid = (year, month, firstWeekday) => {
+  const first = new Date(year, month, 1);
+  const last = new Date(year, month + 1, 0);
+  const firstDayOfWeek = getWeekdayOffsetInGrid(first, firstWeekday);
+  const daysInMonth = last.getDate();
+  const weeks = [];
+  let currentWeek = [];
+  for (let i = 0; i < firstDayOfWeek; i += 1) {
+    currentWeek.push(null);
+  }
+  for (let day = 1; day <= daysInMonth; day += 1) {
+    currentWeek.push(new Date(year, month, day));
+    if (currentWeek.length === DAYS_PER_WEEK) {
+      weeks.push(currentWeek);
+      currentWeek = [];
+    }
+  }
+
+  // Pad end of last week with nulls
+  if (currentWeek.length > 0) {
+    while (currentWeek.length < DAYS_PER_WEEK) {
+      currentWeek.push(null);
+    }
+    weeks.push(currentWeek);
+  }
+  return weeks;
+};
+
+/**
+ * Check if two dates are the same calendar day (ignoring time).
+ */
+export const isSameDay = (a, b) => {
+  if (a === null || b === null) return false;
+  return normalizeDate(a) === normalizeDate(b);
+};
+
+/**
+ * Check if `date` is between `start` and `end` (exclusive), ignoring time.
+ */
+export const isDateInRange = (date, start, end) => {
+  if (start === null) return false;
+  const normalizedDateTime = normalizeDate(date);
+  const normalizedStartDateTime = normalizeDate(start);
+  const normalizedEndDateTime = end !== null ? normalizeDate(end) : normalizedStartDateTime;
+  const low = Math.min(normalizedStartDateTime, normalizedEndDateTime);
+  const high = Math.max(normalizedStartDateTime, normalizedEndDateTime);
+  return normalizedDateTime > low && normalizedDateTime < high;
+};
+
+/**
+ * Check if `date` is in the `disabledDates` list (by calendar day).
+ */
+export const isDateDisabled = (date, disabledDates = []) => {
+  return disabledDates.some(d => isSameDay(date, d));
+};
+
+/** One visible day in the month grid with its row (for Home/End and keyboard nav). */
+
+/** Flat list of dates in grid order (row-major, non-null only) with row index for Home/End */
+export const getDatesWithRow = weeks => {
+  const result = [];
+  weeks.forEach((week, rowIndex) => {
+    week.forEach(date => {
+      if (date !== null) result.push({
+        date,
+        rowIndex
+      });
+    });
+  });
+  return result;
+};
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