There is a setting in /admin/site_settings/category/user_preferences called "default other new topic duration minutes" with some time-based options - I'd like the option to set this to "created in the last 6 months" and to "created in the last 3 months" (our is a relatively slow-moving Discourse instance) - currently the longest time-based setting is "created in the last 2 weeks".
Discuss this on our forum.