Mongodb shell new Date(string) - mongodb

Question

I am experiencing a very weird issue with MongoDB shell version: 2.4.6. It has to do with creating ISODate objects from strings. See below for a specific example.
Why does this not work.
collection.aggregate({$project: {created_at: 1, ts: {$add: new Date('created_at')}}}, {$limit: 1})
{
"result" : [
{
"_id" : ObjectId("522ff3b075e90018b2e2dfc4"),
"created_at" : "Wed Sep 11 04:38:08 +0000 2013",
"ts" : ISODate("0NaN-NaN-NaNTNaN:NaN:NaNZ")
}
],
"ok" : 1
}
But this does.
collection.aggregate({$project: {created_at: 1, ts: {$add: new Date('Wed Sep 11 04:38:08 +0000 2013')}}}, {$limit: 1})
{
"result" : [
{
"_id" : ObjectId("522ff3b075e90018b2e2dfc4"),
"created_at" : "Wed Sep 11 04:38:08 +0000 2013",
"ts" : ISODate("2013-09-11T04:38:08Z")
}
],
"ok" : 1
}

Answer 1

The short answer is that you're passing the string 'created_at' to the Date constructor. If you pass a malformed date string to the constructor, you get ISODate("0NaN-NaN-NaNTNaN:NaN:NaNZ") in return.
To properly create a new date you'd have to do so by passing in the contents of 'created_at'. Unfortunately, I don't know of a way to run a date constructor on a string using the aggregation framework at this time. If your collection is small enough, you could do this in the client by iterating over your collection and adding a new date field to each document.

Answer 2

Kindof similar problem:
I had documents in MongoDB with NO dates being set in, and the DB is filling up so ultimately I need to go in and delete items that are older than one year.
I did sortof have date in a crappy human readable string format. which I can grab from '$ParentMasterItem.Name';
Example: '20211109 VendorName ProductType Pricing Update Workflow'.
So here's my attempt to pull out the dates (via substring parsing) -- (thankfully I do happen to know that every one of the 100K documents has it set the same way)
db.MyCollectionName.aggregate({$project: {
created_at: 1,
ts: {$add: {
$dateFromString: {
dateString: {
/* 0123 (year)
45 (month)
67 (day)
# '20211109 blahblah string'*/
$concat: [ { $substr: [ "$ParentMasterItem.Name", 0, 4 ]}, "-",
{ $substr: [ "$ParentMasterItem.Name", 4, 2 ]}, "-",
{ $substr: [ "$ParentMasterItem.Name", 6, 2 ]},
'T01:00:00Z']
}}}}}}, {$limit: 10})
output:
{ _id: 8445390, ts: ISODate("2022-12-19T01:00:00.000Z") },