Emit a DateTime in the Map function of MongoDb

My map function looks like this:

map = function()
{
   day = Date.UTC(this.TimeStamp.getFullYear(), this.TimeStamp.getMon开发者_如何学Cth(), this.TimeStamp.getDate());
   emit({day : day, store_id : this.Store_Id}, {count : 1});
}

TimeStamp is stored as date in the database, like this:

{ "TimeStamp" : "Mon Mar 01 2010 11:58:09 GMT+0000 (BST)", ...}

I need the "day" in the result collection to be stored as a date type as well, but it's stored as long (Epoch ticks) like this:

{ "_id" : { "day" : 1265414400000, "store_id" : 10}, "value" : { "count" : 7 } }

I tried changing the emit to something like this but didn't help:

emit({day : {"$date" : day},...)

Any ideas as to how to do that?

---

Date.utc is going to return miliseconds from epoch. So when you put your data back into the DB, you can use for example:

 new Date(dateAsLong)

and it will be stored as the BSON date format.

earlier than mongo 1.7 it will show up in your hash as:

 "Mon Mar 01 2010 11:58:09 GMT+0000 (BST)"

1.7+ it will appear as:

 ISODate("2010-03-01T11:58:09Z")