How do I find date difference based on a field value in MongoDB?

I want to find the date difference in a collection but for the objects which matches a given condition. Any clue on how to achieve this.

Currently, I am using a query:

db.sales.aggregate([ {
    $project: {
       item: 1,
       dateDifference: {
          $subtract: [ "$enddate", "$startdate" ]
       }
    }
} ])

but this will return the date difference for all of my objects in the collection.

I want to have something like say I have a items field and there are multiple items say Car, Bike, Cycle etc. Now i only want the date difference based upon the item value.

say we have 4 sessions in a collection with properties like:

[
  {
    _id: "112233",
    item: "CAR",
    startdate: ISODate("2022-03-16T07:38:08.466Z"),
    enddate: ISODate("2022-03-16T08:38:08.466Z")
  },
  {
    _id: "11222333",
    item: "BIKE",
    startdate: ISODate("2022-02-16T07:38:08.466Z"),
    enddate: ISODate("2022-02-14T08:38:08.466Z")
  },
  {
    _id: "1122333243",
    item: "CAR",
    startdate: ISODate("2022-01-16T07:38:08.466Z"),
    enddate: ISODate("2022-02-16T01:38:08.466Z")
  },
  {
    _id: "12312233",
    item: "BUS",
    startdate: ISODate("2021-03-16T07:38:08.466Z"),
    enddate: ISODate("2021-03-16T08:38:08.466Z")
  }
]

Now i want to find the difference of startdate and enddate say for CAR only.

Use $expr and $dateDiff

db.collection.aggregate([
  {
    $match: {
      $expr: {
        $gt: [
          {
            $dateDiff: {
              startDate: "$startdate",
              endDate: "$enddate",
              unit: "minute"
            }
          },
          20
        ]
      }
    }
  }
])

mongoplayground

db.collection.aggregate([
  {
    $match: {
      item: "CAR"
    }
  },
  {
    $set: {
      diff: {
        $dateDiff: {
          startDate: "$startdate",
          endDate: "$enddate",
          unit: "day"
        }
      }
    }
  }
])

mongoplayground