Emit a DateTime in the Map function of MongoDb - mongodb

Question

My map function looks like this:
map = function()
{
day = Date.UTC(this.TimeStamp.getFullYear(), this.TimeStamp.getMonth(), 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?

Answer 1

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")