Question: I need to parse date value to specific format without using format field in dateFromString operator. 

[Mongo Playground](https://mongoplayground.net/p/U9thsJCE88V)

Current situation : 
in Mongodb 4.0 if I format dateString using below it code it give me mentioned output. 

```
 parsedDate: {
        $dateFromString: {
          dateString: "$dateS",
          format: format: "%Y-%m-%dT%H"
        }
      }
Output: "parsedDate": ISODate("2020-01-16T08:00:00Z")

```

I cannot use format field in 3.6 since its not supported.
How do I convert my date to
 `format: "%Y-%m-%dT%H"` in 3.6?

Answer: 
I think following query will solve your problem because MongoDB 3.6 supports *dateFromString*

Input:

```
[
  {
    "dateS": "2020-01-16T08"
  }
]

```

Query:

```
db.collection.aggregate([
  {
    "$project": {
      "year": {"$substr": ["$dateS",0,4]},
      "month": {"$substr": ["$dateS",5,2]},
      "day": {"$substr": ["$dateS",8,2]},
      "hour": {"$substr": ["$dateS",11,2]}
    }
  },
  {
    $addFields: {
      "isoString": {
        "$concat": ["$year","-","$month","-","$day","T","$hour",":00:00Z"]
      }
    }
  },
  {
    $addFields: {
      "newDate": {
        $dateFromString: {"dateString": "$isoString"}
      }
    }
  }
])

```

Output:

```
[
  {
    "_id": ObjectId("5a934e000102030405000000"),
    "day": "16",
    "hour": "08",
    "isoString": "2020-01-16T08:00:00Z",
    "month": "01",
    "newDate": ISODate("2020-01-16T08:00:00Z"),
    "year": "2020"
  }
]

```
This worked for me: I used combined solution from @ambienBeing and @techstack. Thanks 

[Mongo playground](https://mongoplayground.net/p/aUBwxfZO_Qm)

```
  "parsedDate": {
          "$dateFromParts": {
            "year": {
              "$year": {
                $dateFromString: {
                  dateString: {
                    "$concat": [
                      "$_id.dateHour",
                      ":00:00Z"
                    ]
                  }
                }
              }
            },
            "month": {
              "$month": {
                $dateFromString: {
                  dateString: {
                    "$concat": [
                      "$_id.dateHour",
                      ":00:00Z"
                    ]
                  }
                }
              }
            },
            "day": {
              "$dayOfMonth": {
                $dateFromString: {
                  dateString: {
                    "$concat": [
                      "$_id.dateHour",
                      ":00:00Z"
                    ]
                  }
                }
              }
            },
            "hour": {
              "$hour": {
                $dateFromString: {
                  dateString: {
                    "$concat": [
                      "$_id.dateHour",
                      ":00:00Z"
                    ]
                  }
                }
              }
            }
          }
        },

```