fix: parse holiday date strings as local time to prevent timezone shift

When holiday dates are provided as ISO date strings (YYYY-MM-DD), the
previous implementation used `new Date(string)` which parses them as UTC
midnight. This caused holidays to display on the wrong day in timezones
west of UTC.

For example, "2025-01-01" would be parsed as 2025-01-01T00:00:00.000Z,
which when formatted in PST (UTC-8) becomes December 31st, 2024.

This fix uses the existing `parseDate` utility with the ISO format,
which uses date-fns's `parse` function that treats dates as local time.

Fixes #6105

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: martijnrusschen

src/index.tsx

@@ -410,10 +410,12 @@ export class DatePicker extends Component<DatePickerProps, DatePickerState> {
           : newDate();
 
   // Convert the date from string format to standard Date format
+  // Uses parseDate with ISO format to parse as local time, preventing
+  // dates from shifting in timezones west of UTC. See issue #6105.
   modifyHolidays = () =>
     this.props.holidays?.reduce<HolidayItem[]>((accumulator, holiday) => {
-      const date = new Date(holiday.date);
-      if (!isValid(date)) {
+      const date = parseDate(holiday.date, "yyyy-MM-dd", undefined, false);
+      if (!date) {
         return accumulator;
       }
 

src/test/datepicker_test.test.tsx

@@ -5461,6 +5461,40 @@ describe("DatePicker", () => {
       expect(container.querySelector(".react-datepicker")).not.toBeNull();
     });
 
+    it("should apply holiday class to correct date regardless of timezone (issue #6105)", () => {
+      // This test verifies that holidays specified as ISO date strings (YYYY-MM-DD)
+      // are displayed on the correct date. The bug was that new Date("YYYY-MM-DD")
+      // parses as UTC midnight, causing dates to shift in western timezones.
+      const holidays = [{ date: "2024-01-15", holidayName: "Test Holiday" }];
+
+      const { container } = render(
+        <DatePicker
+          selected={new Date(2024, 0, 15)} // January 15, 2024 in local time
+          onChange={() => {}}
+          holidays={holidays}
+          inline
+        />,
+      );
+
+      // Find the day element for January 15th and verify it has the holiday class
+      const jan15 = container.querySelector(
+        ".react-datepicker__day--015:not(.react-datepicker__day--outside-month)",
+      );
+      expect(jan15).not.toBeNull();
+      expect(jan15?.classList.contains("react-datepicker__day--holidays")).toBe(
+        true,
+      );
+
+      // Verify January 14th does NOT have the holiday class (the bug would show it here)
+      const jan14 = container.querySelector(
+        ".react-datepicker__day--014:not(.react-datepicker__day--outside-month)",
+      );
+      expect(jan14).not.toBeNull();
+      expect(jan14?.classList.contains("react-datepicker__day--holidays")).toBe(
+        false,
+      );
+    });
+
     it("should handle deferFocusInput and cancelFocusInput", () => {
       jest.useFakeTimers();