{
    "_id" : "6b109972c9bd9d16a09b70b96686f691bfe2f9b6",
     "history" : [
        {
            "dtEntry" : 1428929906,
            "type" : "I",
            "refname" : "ref1"
        },
        {
            "dtEntry" : 1429082064,
            "type" : "U",
            "refname" : "ref1"
        }
    ],
    "c" : "SomeVal",
    "p" : "anotherVal"
}

db.mydataset.aggregate({$project:{c:1,p:1,type:"$history.type",DtEntry:"$history.dtEntry",dater:{$dayOfMonth:new Date(DtEntry)}}})

$dayOfMonth:new Date(DtEntry)
$dayOfMonth:new Date(history.DtEntry)
$dayOfMonth:new Date("history.DtEntry")
$dayOfMonth:new Date("$history.DtEntry")