combine milestone start and end dates in collapsed sidebar

app/assets/stylesheets/pages/milestone.scss

@@ -68,6 +68,18 @@
     }
   }
 
+  .collapsed-milestone-date {
+    font-size: 12px;
+  }
+
+  .milestone-date {
+    display: block;
+  }
+
+  .date-separator {
+    line-height: 5px;
+  }
+
   .remaining-days strong {
     font-weight: normal;
   }

app/views/shared/milestones/_sidebar.html.haml

@@ -17,16 +17,12 @@
       .value.hide-collapsed
         = milestone_progress_bar(milestone)
 
-    .block.start_date
-      .sidebar-collapsed-icon
-        = icon('calendar', 'aria-hidden': 'true')
-        %span.js-due-date-sidebar-value
-          = milestone.start_date.try(:to_s, :medium) || 'None'
-      .title.hide-collapsed
+    .block.start_date.hide-collapsed
+      .title
         Start date
         - if @project && can?(current_user, :admin_milestone, @project)
           = link_to 'Edit', edit_namespace_project_milestone_path(@project.namespace, @project, @milestone), class: 'edit-link pull-right'
-      .value.hide-collapsed
+      .value
         %span.value-content
           - if milestone.start_date
             %span.bold= milestone.start_date.to_s(:medium)
@@ -36,8 +32,22 @@
     .block.due_date
       .sidebar-collapsed-icon
         = icon('calendar', 'aria-hidden': 'true')
-        %span.js-due-date-sidebar-value
-          = milestone.due_date.try(:to_s, :medium) || 'None'
+        %span.collapsed-milestone-date
+          - if milestone.start_date && milestone.due_date
+            - if milestone.start_date.year == milestone.due_date.year
+              .milestone-date= milestone.start_date.strftime('%b %-d')
+            - else
+              .milestone-date= milestone.start_date.strftime('%b %-d %Y')
+            .date-separator -
+            .due_date= milestone.due_date.strftime('%b %-d %Y')
+          - elsif milestone.start_date
+            From
+            .milestone-date= milestone.start_date.strftime('%b %-d %Y')
+          - elsif milestone.due_date
+            Until
+            .milestone-date= milestone.due_date.strftime('%b %-d %Y')
+          - else
+            None
       .title.hide-collapsed
         Due date
         - if @project && can?(current_user, :admin_milestone, @project)