There are no links to the user manual, where is it? On Meta?
Topic on Talk:Wikibase
Unless I'm blind and all links on this wiki and Meta are wrong, it seems there is no user manual.
What I was looking for was Wikibase/DataModel#Dates_and_times, approximately (bugzilla:64593). It took me 25 minutes to find that page/section and from the warning at the top of the page it seems to be an implementation plan, not documentation. I think this extension really needs to have a user manual, which would typically be in Help namespace here on mediawiki.org.
If most of the planned stuff has been implemented, you could rephrase most of the pages and then mark the various sections with version markers as we do for the rest of MediaWiki, also tagging stuff left to future releases.