Prøv dette:
<% if @user.created_at > 5.days.ago.beginning_of_day %>
<span>New User!</span>
<% end%>
Eller du kan bruge:
<%= time_ago_in_words(@user.created_at) %>
Kilde:https://apidock.com/rails/ActionView/Helpers/DateHelper/time_ago_in_words