save current date before Pikaday init

app/assets/javascripts/due_date_select.js

@@ -170,6 +170,8 @@ export default class DueDateSelectors {
   initMilestoneDatePicker() {
     $('.datepicker').each(function initPikadayMilestone() {
       const $datePicker = $(this);
+      const datePickerVal = $datePicker.val();
+
       const calendar = new Pikaday({
         field: $datePicker.get(0),
         theme: 'gitlab-theme animate-picker',
@@ -182,7 +184,7 @@ export default class DueDateSelectors {
         },
       });
 
-      calendar.setDate(parsePikadayDate($datePicker.val()));
+      calendar.setDate(parsePikadayDate(datePickerVal));
 
       $datePicker.data('pikaday', calendar);
     });

changelogs/unreleased/41671-fixing-milestone-date-change-when-editing.yml

+---
+title: "Fixing milestone date change when editing"
+merge_request: 20279
+author: Orlando Del Aguila
+type: fixed
\ No newline at end of file

spec/features/milestones/user_edits_milestone_spec.rb

+require "rails_helper"
+
+describe "User edits milestone", :js do
+  set(:user) { create(:user) }
+  set(:project) { create(:project) }
+  set(:milestone) { create(:milestone, project: project, start_date: Date.today, due_date: 5.days.from_now) }
+
+  before do
+    project.add_developer(user)
+    sign_in(user)
+
+    visit(edit_project_milestone_path(project, milestone))
+  end
+
+  it "shows the right start date and due date" do
+    start_date = milestone.start_date.strftime("%F")
+    due_date = milestone.due_date.strftime("%F")
+
+    expect(page).to have_field(with: start_date)
+    expect(page).to have_field(with: due_date)
+  end
+end