Actually, this appears to be a WordPress bug.
His code is requesting the modified time of the file and assigning to $ft using PHP's filemtime() function.
He then asks PHP format the date and time using the date() function and WordPress' get_option function:
date(get_option('date_format'),$ft) . " " . date(get_option('time_format'),$ft)
The PHP date() function should read the default timezone and report the proper time, but instead it's returning the UTC time as evidenced by examining the output of date_default_timezone_get() - it's showing 'UTC' even though the blog is set to 'America/New_York'...
This means WordPress is not setting the default timezone in time or at all when the plugin runs...
No?