The plugin returns Header "Last-Modified: Thu, 01 Jan 1970 00:00:00 GMT"

Hi, it is working fine here on my end, but again I will re test it and update the plugin in needed.

Just tested it out. Take a look: https://i.imgur.com/xBKf0zO.jpg

WordPress v.4.2.2

Have a look at these…

https://www.my-soul.net/blog/ > https://postimg.org/image/mb1e7ryx7/
https://www.my-soul.net/blog/их-две-времени-программ/ > https://postimg.org/image/fk6wdv2ql/

or you can try the links your self.

So the screenshot you have provided i.e. – https://postimg.org/image/mb1e7ryx7/ showing If modified header isd working fine and date time is also right.

But for this – https://postimg.org/image/fk6wdv2ql/ – I see the issue. I think this might be happening due to the non english letters in your url. Can you please create an english test post which will have a normal english url and then try it out. Please let me know your finding.

This is a post with English slug https://www.my-soul.net/blog/this-is-another-post/ > https://postimg.org/image/z4t2zza2j/

This is a page with English slug https://www.my-soul.net/blog/services-2/ > https://postimg.org/image/jdyfcdgi1/

I think I got the issue. Will try to fix it if I get some free time. Thanks for reporting it. One last question. Did you installed WP in a sub directory or you set your permalink to show up the /blog/ ?

It is installed in a sub directory (/blog).