comparison of Time with -279519369 failed (ArgumentError)

/usr/local/httpd/htdocs/mini/diary/tdiary.rb:1447:in `<'
/usr/local/httpd/htdocs/mini/diary/tdiary.rb:1447:in `last_modified'
/usr/local/httpd/htdocs/mini/diary/tdiary.rb:1445:in `last_modified'
/usr/local/httpd/htdocs/mini/diary/index.rb:59