db.multikey.insert({accounts:[1, 2], day:ISODate('2013-01-01')})
db.multikey.insert({accounts:[1, 2], day:ISODate('2013-01-02')})
db.multikey.insert({accounts:[1, 2], day:ISODate('2013-01-03')})
db.multikey.ensureIndex({accounts:1, day:1})
db.multikey.find({accounts:1, day:{$gte: ISODate('2013-01-01'), $lt: ISODate('2013-01-02')}}).explain()
{
        "cursor" : "BtreeCursor accounts_1_day_1",
        "isMultiKey" : true,
        "n" : 1,
        "nscannedObjects" : 3,
        "nscanned" : 3,
        "nscannedObjectsAllPlans" : 3,
        "nscannedAllPlans" : 3,
        "scanAndOrder" : false,
        "indexOnly" : false,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "millis" : 0,
        "indexBounds" : {
                "accounts" : [
                        [
                                1,
                                1
                        ]
                ],
                "day" : [
                        [
                                ISODate("2013-01-01T00:00:00Z"),
                                ISODate("292278995-01--2147483647T07:12:56.808Z")
                        ]
                ]
        },
        "server" : "vagrant-ubuntu:27017"
}