It is impossible to access Q-Id info with a Lua function. It can be done only by a API call like

https://www.wikidata.org/w/api.php?action=wbgetentities&ids=Q42&props=info

It returns:

{
    "entities":  {
        "Q42":  {
            "pageid":  138,
            "ns":  0,
            "title":  "Q42",
            "lastrevid":  503604471,
            "modified":  "2017-06-20T10:01:26Z",
            "type":  "item",
            "id":  "Q42"
        }
    },
    "success":  1
}

The most important is of course the modified parameter. I think a function should return the complete info block.