This can be easily done using Events, on Document.New

Just create a new class (eg. UmbracoEvents.cs)

using System;
using umbraco.BusinessLogic;
using umbraco.cms.businesslogic.web;
using Examine;

public class UmbracoEvents: ApplicationBase
{
  /// <summary>Constructor</summary>
  public UmbracoEvents()
  {
    Document.New += new Document.NewEventHandler(Document_New);
  }

  private void Document_New(Document sender, umbraco.cms.businesslogic.NewEventArgs e)
  {
    if (sender.ContentType.Alias == "News")
    {
      sender.getProperty("date").Value = DateTime.Today; // Set the date for a new News item to today
    }
  }
}

For More Understanding please refer below link

https://our.umbraco.com/Documentation/Getting-Started/Code/Subscribing-To-Events/