We used to have unlimited editing and deleting for all users on the forum. However you can thank the users that abused that privilege in the past. One such user, a known ban dodger, would actually edit all his posts to include doxxing information, and images of gore (such as people getting bifurcated, a guy blowing his head off with a shotgun, a bunch of dogs getting ran over by a bus, literal dead children, etc.) This troll would post normally, and then days later edit his posts to try and keep the doxing information/gore on the forum as long as possible. So unfortunately, you are now not able to edit/delete your posts once the timeframe has expired. The only exception is if you reach Zoo Settler rank or higher, 150+ counted messages on the forum. Then you'll get the ability to delete/edit posts with no time limits.
The easiest solution is to report your own posts to the moderators by hitting the "Report" button at the bottom for your posts. Just make the report reason "Please delete my post." and usually it'll be deleted within 5 minutes to at most a few hours. As we don't have issues with self requested post deletions for most cases.
