How can I index a date field with the default value of "now"?

I need to add a date field type such that the document will take the current system date time as the default value. I'm using Elasticsearch 7.5.

PUT /myindex/_mappings
{
     "properties": {    
       "create_date": {
         "type": "date",
         "format": "yyyy-MM-dd HH:mm:ss",
         "null_value": "now"
       }
     }
}

There's no such functionality in Elasticsearch. At least not directly.

What you can do, however, is create an ingest pipeline that assigns the current datetime from inside a script processor:

PUT _ingest/pipeline/auto_now_add
{
  "description": "Assigns the current date if not yet present",
  "processors": [
    {
      "script": {
        "source": """
          // don't overwrite if present
          if (ctx['create_date'] == null) {
            ctx['create_date'] = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date());
          }
        """
      }
    }
  ]
}

After that, as you PUT your index, you'll specify the default_pipeline:

PUT myindex
{
  "settings": {
    "index": {
      "default_pipeline": "auto_now_add"     <---
    }
  },
  "mappings": {
    "properties": {
      "create_date": {
        "type": "date",
        "format": "yyyy-MM-dd HH:mm:ss"
      }
    }
  }
}

Then, as you insert a doc with a missing create_date:

PUT myindex/_doc/1
{
  "abc": "def"
}

Elasticsearch will have auto-added the current timestamp. Verify with:

POST myindex/_search