Aggregation pipeline to convert ISODate to desired string format in MongoDB prior to v3.0

Convert Date Format with MongoDB Aggregation.markdown

Convert Date Format with MongoDB Aggregation

The following aggregation pipeline is used to convert ISODate to desired string format before MongoDB started providing $dateToStringoperator in v3.0

{
  $project: {
    day: { $dayOfMonth: "$LastUpdate" },
    month: { $month: "$LastUpdate" },
    year: { $year: "$LastUpdate" },
  }
}, {
  $project: {
    day: {
      $cond: {
        if: { $lt: [ "$day", 10 ] },
        then: {
          $concat: [
            "0",
            { $substr: [ "$day", 0, 2 ] }
          ]
        },
        else: { $substr: [ "$day", 0, 2 ] }
      }
    },
    month: {
      $cond: {
        if: { $lt: [ "$month", 10 ] },
        then: {
          $concat: [
            "0",
            { $substr: [ "$month", 0, 2 ] }
          ]
        },
        else: { $substr: [ "$month", 0, 2 ] }
      }
    },
    year: { $substr: [ "$year", 0, 4 ] },
  }
}, {
  $project: {
    date: {
      $concat: [
        { $substr: [ "$year", 0, 4 ] },
        "-",
        { $substr: [ "$month", 0, 2 ] },
        "-",
        { $substr: [ "$day", 0, 2 ] }
      ]
    }
  }
}, {
  $group: {
    _id: "$date",
    count: { $sum: 1 }
  }
}

It's a modification from this Stack Overflow's answer.