How to group by seconds without the ISODate decimal part in MongoDB

How to group by seconds without the ISODate decimal part in MongoDB

I wanted to query the database in order to find the number of post per
second to feed into a graph to show activity trend. I use
spring-data-mongo but for now, the first step is to do this in the mongo
shell before worrying about how to do from java.
I used the aggregation framework on it as shown below:
db.post.group({
key:{dateCreated: 1},
cond: { dateCreated:
{
"$gt": new ISODate("2013-08-09T05:51:15Z"),
"$lt": new ISODate("2013-08-09T05:51:20Z")
}
},
reduce: function(cur, result){
result.count += 1
},
initial: {count:0}
})
The result is encouraging but is seems because of the decimal part of the
ISODate, the count seems wrong as it does group per seconds with the
decimal making each count 1.
[
{
"dateCreated" : ISODate("2013-08-09T05:51:15.332Z"),
"count" : 1
},
{
"dateCreated" : ISODate("2013-08-09T05:51:15.378Z"),
"count" : 1
},
{
"dateCreated" : ISODate("2013-08-09T05:51:15.377Z"),
"count" : 1
},
// many more here
]
Is there a way to just consider only the seconds part as in result like
below:
[
{
"dateCreated" : ISODate("2013-08-09T05:51:15Z"),
"count" : 5
},
{
"dateCreated" : ISODate("2013-08-09T05:51:16Z"),
"count" : 8
},
{
"dateCreated" : ISODate("2013-08-09T05:51:17Z"),
"count" : 3
},
{
"dateCreated" : ISODate("2013-08-09T05:51:18Z"),
"count" : 10
},
{
"dateCreated" : ISODate("2013-08-09T05:51:19Z"),
"count" : 2
},
{
"dateCreated" : ISODate("2013-08-09T05:51:20Z"),
"count" : 13
}
]
Thank for reading this.