Code stolen from the real author and doesn't work in a proper way

This plugin is made with a piece of code given by an author named Cybmeta to Saumya Majumder as you can verify here : https://wordpress.stackexchange.com/questions/172966/if-modified-since-http-header

2nd issue : it doesn’t work in a proper manner, the date/time format is wrong, the result is always 200 ok even if the page haven’t change (correct answer is 304 not modified in this case).