{
"installAt" : "2016-08-01T10:24:38.502Z",
"success" : true
"app" : "web"
}

db.sampleCollection.aggregate(
       [
          {
            $group : {
               _id : { month: { $month: "$installAt" }, day: { $dayOfMonth: "$installAt" }, year: { $year: "$installAt" } },
               count: { $sum: 1 }
            }
          }
       ]
    )

assert: command failed: {
        "errmsg" : "exception: can't convert from BSON type EOO to Date",
        "code" : 16006,
        "ok" : 0
} : aggregate failed
Error: command failed: {
        "errmsg" : "exception: can't convert from BSON type EOO to Date",
        "code" : 16006,
        "ok" : 0
} : aggregate failed
    at Error (<anonymous>)
    at doassert (src/mongo/shell/assert.js:11:14)
    at Function.assert.commandWorked (src/mongo/shell/assert.js:244:5)
    at DBCollection.aggregate (src/mongo/shell/collection.js:1149:12)
    at (shell):1:23
2016-12-07T19:48:25.541+0530 Error: command failed: {
        "errmsg" : "exception: can't convert from BSON type EOO to Date",
        "code" : 16006,
        "ok" : 0
} : aggregate failed at src/mongo/shell/assert.js:13