The only use case for a date picker I can think of is to set a date in an infobox or other template (such as cleanup templates, which like to know the month + year they were assigned). However, right now in VE templates don't actually have full VE-enabled GUIs for editing their parameters. Just autosuggest search and wikitext input areas. Correct? Another typical use for a date picker, such as marking date of birth on a user profile, is something we don't yet support and which should have Legal involved before we do. 

TL;DR: we'll build things like a date picker when we need them. It's cool to have designs, but also really confusing to put them in mocks or a style guide when they're not used. I'd encourage the UX team to make a special "future/experimental" category that separates out these form types that don't exist yet. This will reduce confused questions. ;) 

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