Merge branch 'fix-date-off-by-1' into 'master'

Fix date bug in JS that makes dates not work with strings

Closes #29916

See merge request !12147

app/assets/javascripts/due_date_select.js

@@ -2,6 +2,8 @@
 /* global dateFormat */
 /* global Pikaday */
 
+import DateFix from './lib/utils/datefix';
+
 class DueDateSelect {
   constructor({ $dropdown, $loading } = {}) {
     const $dropdownParent = $dropdown.closest('.dropdown');
@@ -43,14 +45,13 @@ class DueDateSelect {
 
   initDatePicker() {
     const $dueDateInput = $(`input[name='${this.fieldName}']`);
-
+    const dateFix = DateFix.dashedFix($dueDateInput.val());
     const calendar = new Pikaday({
       field: $dueDateInput.get(0),
       theme: 'gitlab-theme',
       format: 'yyyy-mm-dd',
       onSelect: (dateText) => {
         const formattedDate = dateFormat(new Date(dateText), 'yyyy-mm-dd');
-
         $dueDateInput.val(formattedDate);
 
         if (this.$dropdown.hasClass('js-issue-boards-due-date')) {
@@ -62,7 +63,7 @@ class DueDateSelect {
       }
     });
 
-    calendar.setDate(new Date($dueDateInput.val()));
+    calendar.setDate(dateFix);
     this.$datePicker.append(calendar.el);
     this.$datePicker.data('pikaday', calendar);
   }
@@ -168,6 +169,7 @@ class DueDateSelectors {
   initMilestoneDatePicker() {
     $('.datepicker').each(function() {
       const $datePicker = $(this);
+      const dateFix = DateFix.dashedFix($datePicker.val());
       const calendar = new Pikaday({
         field: $datePicker.get(0),
         theme: 'gitlab-theme animate-picker',
@@ -177,7 +179,8 @@ class DueDateSelectors {
           $datePicker.val(dateFormat(new Date(dateText), 'yyyy-mm-dd'));
         }
       });
-      calendar.setDate(new Date($datePicker.val()));
+
+      calendar.setDate(dateFix);
 
       $datePicker.data('pikaday', calendar);
     });

app/assets/javascripts/lib/utils/datefix.js

+const DateFix = {
+  dashedFix(val) {
+    const [y, m, d] = val.split('-');
+    return new Date(y, m - 1, d);
+  },
+};
+
+export default DateFix;