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

<%
  day = 366
  current_day = DateTime.current.strftime('%j').to_i
%>
<% comments.each do |comment| %>
  <li class="comment" id="<%= comment.id %>">
  <%
    comment_day = comment.created_at.strftime('%j').to_i

    if comment_day < current_day and comment_day < day then 
    day = comment.created_at.strftime('%j').to_i
  %>
  <p class="text-center"><%= comment.created_at.strftime('%d.%m.%Y') %></p>
  <% end %>
  <%= render 'project_comments/comment.html.erb', comment: comment %></li>
<% end %>