Dynamic MongoDB aggregation using $$NOW

Fellows;

Would anyone be so kind as to shed a light on this matter which has been stumping me for a few days now? It should be something simple, but I can't seem to find where the problem lies...

I need to retrieve one year worth of records from a collection based on today's date. Simply put, if today is 2023-02-15, I need records created after 2022-02-15.

This works, of course:

[
  { 
    $match: { _created: { $gte: ISODate("2022-02-15") } }
  }
]

The problem is that, when trying to create an aggregation that does not need to constantly be updated, $$NOW does not seem to work and I cannot understand why.

For example, this does not work:

[
  {
    $match:
    {
       _created:
       {
         $gte:
         { 
           $subtract: [ "$$NOW", { $multiply: [1000, 60, 60, 24, 365] } ]
         }
       }
    }
  }
]

And this doesn't either:

[
  {
    $set: 
    { 
      startDate: { $subtract: [ "$$NOW", { $multiply: [1000, 60, 60, 24, 365] } ] }
    }
  },
  {
    $match:
    {
      _created: { $gte: ISODate("$startDate") }
    }
  }
]

I have tried moving things around, parsing the result of $$NOW to get just the date part, converting it to different types... The result is always the same: No errors, but no filtered records either.

And I have the feeling the answer might be something really silly I am just overlooking.

Can someone more experienced lend a helping hand?

P.S.: The aggregation continues after the required records are retrieved, I just did not care to list what happens because the problem relates only to the matching of records. Our Mongod version is 6.0.3, I am working on MongoDB Compass and the idea is to save this aggregation as a query which provides answers to an API call.

You need to use $expr and $gte aggregation operator (do not mistake with $gte Query Operator). In MongoDB 5.0 and newer, you can also use Date Expression Operators like $dateSubtract

  {
    $match:
    {
       $expr:
       {
         $gte:
           [ "$_created", { $dateSubtract: { startDate: "$$NOW", unit: "year", amount: 1 } } ]
       }
    }
  }