stance455 reported that the “Reset bump date” feature stopped working despite functioning previously. After adding a reply to a topic, clicking “Reset bump date” had no effect. Moin explained that since the new reply is the last visible post, resetting the bump date does nothing — it only works if the post is edited (which bumps the topic) or deleted, allowing the date to reset to the first post’s time. stance455 thanked Moin and noted that the related feature “Toggle topic bump” from the linked post was exactly what they needed, expressing appreciation for Discourse’s hidden capabilities.
Since your new post is the last visible post reset bump date does not work. It would work if you edit your post, which bumps the topic. Then it resets to the time the post was created. You could also delete ot, then you csn reset the date to match the first post.
Maybe the topic I quoted from is a help for you.