MongoDB query to convert the field value and create datetime day of month during projection?

To convert filed value to create datetime day of month, use MongoDB aggregate(). Let us create a collection with documents −

> db.demo209.insertOne(
...   {
...      "_id" : "101",
...      "details" : [
...         {
...            "dat" : 1528929908,
...            "Name" : "Chris"
...         },
...         {
...            "dat" : 1529082069,
...            "Name":"Carol"
...         }
...      ],
...      "Age" : 25,
...      "CountryName" : "US"
...   }
...);
{ "acknowledged" : true, "insertedId" : "101" }

Display all documents from a collection with the help of find() method −

> db.demo209.find().pretty();

This will produce the following output −

{
   "_id" : "101",
   "details" : [
      {
         "dat" : 1528929908,
         "Name" : "Chris"
      },
      {
         "dat" : 1529082069,
         "Name" : "Carol"
      }
   ],
   "Age" : 25,
   "CountryName" : "US"
}

Following is the query to convert the field value and create datetime day of month during projection −

> db.demo209.aggregate({
...   "$unwind": "$details"
...   }, {
...         "$project": {
...            "Age": 1,
...            "CountryName": 1,
...            "Name": "$details.Name",
...            "DayOfMonth": {
...               "$dayOfMonth": {
...                  "$add": [new Date(0), {
...                     "$multiply": ["$details.dat", 1000]
...               }]
...            }
...         }
...   }
...})

This will produce the following output −

{ "_id" : "101", "Age" : 25, "CountryName" : "US", "Name" : "Chris", "DayOfMonth" : 13 }
{ "_id" : "101", "Age" : 25, "CountryName" : "US", "Name" : "Carol", "DayOfMonth" : 15 }