For those in the same situation. here is how I modified my query. Thanks to @Sammaye.
db.post.aggregate(
{
$match: { dateCreated:
{
"$gt": new ISODate("2013-08-09T05:51:15.000Z"),
"$lt": new ISODate("2013-08-09T05:51:20.000Z")
}
}
},
{
$group: {
_id: {
hour: {$hour: "$dateCreated"},
minute: {$minute: "$dateCreated"},
second: {$second: "$dateCreated"}
},
cnt: {$sum : 1}
}
}
)
{
"result" : [
{
"_id" : {
"hour" : 5,
"minute" : 51,
"second" : 19
},
"cnt" : 26
},
{
"_id" : {
"hour" : 5,
"minute" : 51,
"second" : 18
},
"cnt" : 29
},
{
"_id" : {
"hour" : 5,
"minute" : 51,
"second" : 17
},
"cnt" : 27
},
{
"_id" : {
"hour" : 5,
"minute" : 51,
"second" : 16
},
"cnt" : 25
},
{
"_id" : {
"hour" : 5,
"minute" : 51,
"second" : 15
},
"cnt" : 16
}
],
"ok" : 1
}