There's a time limit on editing posts. Not sure what it is but there it is. On the old board one could delete a post as long as no one had responded to it and editing could be done at any time. Lemme check on deleting.

Edit: There's no delete button but one can always edit it down to nothing.

Re-edit: B'lay m'last. There is a delete option within the edit screen.

One last edit: The Mark as Edited option can be unchosen even after it has been chosen in a previous edit.