NoMethodError in Post#index

Showing app/views/post/index.html.erb where line #10 raised: 

You have a nil object when you didn't expect it!
The error occurred while evaluating nil.strftime

Extracted source (around line #10): 

7: 
8:   <div class="sidebar">
9:     <%= render :partial => "search" %>
10:     <%= print_advertisement("vertical") %>
11:     <div style="margin-bottom: 1em;">
12:       <h5>Mode</h5>
13:       <form onsubmit="return false;" action="">

RAILS_ROOT: /var/www/foxbox

Application Trace | Framework Trace | Full Trace 
/var/www/foxbox/app/models/advertisement.rb:14:in `date_path'
/var/www/foxbox/app/models/advertisement.rb:18:in `image_url'
/var/www/foxbox/app/helpers/advertisement_helper.rb:5:in `print_advertisement'
/var/www/foxbox/app/views/post/index.html.erb:10:in `_run_erb_app47views47post47index46html46erb'

Request

Parameters: 

None

Show session dump

Response

Headers: 

{"Content-Type"=>"text/html",
 "Set-Cookie"=>"blacklisted_tags=; path=/",
 "Cache-Control"=>"no-cache"}