You need to post a new update message for that time to change. Normally, when doing a release, you'd have an announcement with it of some sort.
As this is basically intentional, though I appreciate that it's not 100% intuitive. The alternative potentially has people hunting down update info in multiple places to figure out why it changed: now it's just when you post an "update" (distinct from a "version" as an update doesn't have to be a new release).