Charts don't work with saved String date

[chris (Chris Dellaway)]

Date() returns a string, new Date() a Date object.

Date - JavaScript | MDN

As an exercise I converted the date string back to a date.

You could use this part as a project or for an update command etc… There probably is a better way to do it.

db.foo.find(
{},
{
  "createdAt": {
    "$toDate": {
      "$reduce": {
        "input": {
          "$slice": [
            {
              "$split": [
                "$createdAt",
                " "
              ]
            },
            6
          ]
        },
        "initialValue": "",
        "in": {
          "$concat": [
            "$$value",
            " ",
            "$$this"
          ]
        }
      }
    }
  }
}