I believe Reset Bump Date will change the Topic Time Stamp back to the last visible Post submission date if you delete a Post.
Obviously if you add a new Post that will represent the new Date/Time. So not what you want here.
I think what you need is to change the Topic Timestamp.
Try using the associated control on the Topic (not Post) wrench (at the foot of the Topic) to change the Timestamp to the date of the OP.
NB Be aware I believe it will change all Post Time Stamps on the Topic.