scinote-web/app/views/activities/_list.html.erb

<% if activities.first.created_at.to_date == Date.today %>
  <li class="text-center activity-date-item">
    <span class="label label-primary">
      <%=t "activities.index.today" %>
    </span>
  </li>
<% end %>
<% activities.each_with_index do |activity, index| %>
  <% prevDate = activities[index - 1] ? activities[index - 1].created_at.to_date :  Date.new(1901, 1, 1) %>
  <% if activity.created_at.to_date < prevDate %>
    <li class="text-center activity-date-item">
      <span class="label label-primary">
        <%= activity.created_at.strftime('%d.%m.%Y') %>
      </span>
    </li>
  <% end %>
  <%= render 'activities/activity.html.erb', activity: activity %>
<% end %>