templatefilters: declare input type as date where appropriate I'm not sure if the templateutil.date type can be a thing. Currently it's just a constant.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}