Thank you for maintaining this plugin, it is very handy for my personal site. I know awhile back I created a topic about timestamps in the activity log and it never was working for me so I had to manually adjust the php file. I'm hoping we can give it another shot.
My activity is being converted to a time in the future equal to my gmt offset and I think I know why. The function strtotime() uses the timezone defined in date_default_timezone_set() but time() does not. When you try to find the difference on line 833 you are comparing two different timezones.
Line 830 should look like this to take into consideration installs with the date_default_timezone already set:
$in_seconds = strtotime($posted_date) + ($gmt_offset*3600);