Ok, I have one more idea:
the module is using the strtotime PHP function. I went to the API and read that the function actually uses the default time zone of the PHP.ini file (I thought it was using the date.timezone). I saw that your default time zone is set to 'UTC', not 'Europe/Athens'. That may be the problem. Try changing the default time zone. Hopefully that will do it.

I am not sure what is wrong with your configuration, so you can try this fix for your site:
in the module's directory, go to /tmpl/default.php.
Try and change the line 113 to:It eliminates the PHP function strtotime which gives problem here. The hours should reflect the ones you see on K2. Please let me know how that works for you.

I am sorry, but I am out of options here.
Now that I think about it, I made you change the regular file, not the K2 one. Are your articles coming from K2?
If so, do the same as above but in tmpl/k2.php.