Use title attribute instead of data-original-title.

app/assets/javascripts/milestone_select.js.coffee

@@ -25,7 +25,7 @@ class @MilestoneSelect
       if issueUpdateURL
         milestoneLinkTemplate = _.template(
           '<a href="/<%= namespace %>/<%= path %>/milestones/<%= iid %>">
-            <span class="has-tooltip" data-container="body" data-original-title="<%= remaining %>">
+            <span class="has-tooltip" data-container="body" data-title="<%= remaining %>">
               <%= _.escape(title) %>
             </span>
           </a>'
@@ -34,7 +34,7 @@ class @MilestoneSelect
         milestoneLinkNoneTemplate = '<div class="light">None</div>'
 
         collapsedSidebarLabelTemplate = _.template(
-          '<span class="has-tooltip" data-container="body" data-original-title="<%= remaining %>" data-placement="left">
+          '<span class="has-tooltip" data-container="body" data-title="<%= remaining %>" data-placement="left">
             <%= _.escape(title) %>
           </span>'
         )

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

@@ -56,7 +56,7 @@
           = icon('clock-o')
           %span
             - if issuable.milestone
-              %span.has-tooltip{"data-container" => "body", "data-placement" => "left", "data-original-title" => milestone_remaining_days(issuable.milestone, false)}
+              %span.has-tooltip{title: milestone_remaining_days(issuable.milestone), data: {container: 'body', html: 1, placement: 'left'}}
                 = issuable.milestone.title
             - else
               None
@@ -68,7 +68,7 @@
         .value.bold.hide-collapsed
           - if issuable.milestone
             = link_to namespace_project_milestone_path(@project.namespace, @project, issuable.milestone) do
-              %span.has-tooltip{"data-container" => "body", "data-original-title" => milestone_remaining_days(issuable.milestone, false)}
+              %span.has-tooltip{ "title" => milestone_remaining_days(issuable.milestone), data: {container: 'body', html: 1 } }
                 = issuable.milestone.title
           - else
             .light None