Fix display bug when selecting a date range

src/js/datetime.ts

@@ -938,7 +938,12 @@ export class DateTime extends Date {
       const y = year || dt.getFullYear();
       let M = 0;
       if (!(year && !month)) {
-        M = month > 0 ? month - 1 : dt.getMonth();
+        M =
+          month > 0
+            ? localization.monthZeroIndex
+              ? month
+              : month - 1
+            : dt.getMonth();
       }
       if (zone) {
         return new DateTime(

src/js/display/calendar/date-display.ts

@@ -179,6 +179,7 @@ export default class DateDisplay {
       // format the string to a date
       const innerDate = DateTime.fromString(attributeValue, {
         format: 'yyyy-MM-dd',
+        monthZeroIndex: true,
       });
 
       // find the position of the target in the date container
@@ -192,7 +193,7 @@ export default class DateDisplay {
 
       //format the start date so that it can be found by the attribute
       const rangeStartFormatted = this._dateToDataValue(rangeStart);
-      const rangeStartIndex = allDays.findIndex(
+      let rangeStartIndex = allDays.findIndex(
         (e) => e.getAttribute('data-value') === rangeStartFormatted
       );
       const rangeStartElement = allDays[rangeStartIndex];
@@ -214,6 +215,9 @@ export default class DateDisplay {
       let lambda: (_, index) => boolean;
 
       if (innerDate.isBefore(rangeStart)) {
+        if (rangeStartElement === undefined) {
+          rangeStartIndex = Number.MAX_SAFE_INTEGER;
+        }
         currentTarget.classList.add(Namespace.css.rangeStart);
         rangeStartElement?.classList.remove(Namespace.css.rangeStart);
         rangeStartElement?.classList.add(Namespace.css.rangeEnd);

src/js/utilities/options.ts

@@ -77,6 +77,7 @@ export interface FormatLocalization {
   hourCycle?: Intl.LocaleHourCycleKey;
   locale?: string;
   ordinal?: (n: number) => any; //eslint-disable-line @typescript-eslint/no-explicit-any
+  monthZeroIndex?: boolean;
 }
 
 export interface Localization extends FormatLocalization {

test/datetime.test.ts

@@ -75,6 +75,22 @@ test('Can create with string', () => {
   expect(dt.month).toBe(12 - 1); //minus 1 because javascript 🙄
   expect(dt.date).toBe(31);
   expect(dt.year).toBe(2022);
+
+  const zeroIndexMonthDt = DateTime.fromString('2024-11-01', {
+    format: 'yyyy-MM-dd',
+    monthZeroIndex: true,
+  });
+  expect(zeroIndexMonthDt.year).toBe(2024);
+  expect(zeroIndexMonthDt.month).toBe(11);
+  expect(zeroIndexMonthDt.date).toBe(1);
+
+  const nonZeroIndexMonthDt = DateTime.fromString('2024-11-01', {
+    format: 'yyyy-MM-dd',
+    monthZeroIndex: false,
+  });
+  expect(nonZeroIndexMonthDt.year).toBe(2024);
+  expect(nonZeroIndexMonthDt.month).toBe(10);
+  expect(nonZeroIndexMonthDt.date).toBe(1);
 });
 
 test('Can create clone', () => {