headers error when install a plugin

i intalled time_since plugin. it works good at the main page of my site but after activating time_since plugin, i cant log out. when i try to log out, there is an error like ; Warning: Cannot modify header information - headers already sent by (output started at /home/bayrakci/public_html/halil/wp-content/plugins/time_since.php:51) in /home/bayrakci/public_html/halil/wp-login.php on line 167
can anyone help me?
P.S. it isnt a problem for only time_since plugin. when i installed moody plugin, i got the same error.