Setting the default value of a DateTime Property to DateTime.Now inside the System.ComponentModel Default Value Attrbute

You cannot do this with an attribute because they are just meta information generated at compile time. Just add code to the constructor to initialize the date if required, create a trigger and handle missing values in the database, or implement the getter in a way that it returns DateTime.Now if the backing field is not initialized.

public DateTime DateCreated
{
   get
   {
      return this.dateCreated.HasValue
         ? this.dateCreated.Value
         : DateTime.Now;
   }

   set { this.dateCreated = value; }
}

private DateTime? dateCreated = null;

Add below to the DateTime property

[DatabaseGenerated(DatabaseGeneratedOption.Computed)]

There's no reason I can come up with that it shouldn't be possible to do through an attribute. It might be in Microsoft's backlog. Who knows.

The best solution I have found is to use the defaultValueSql parameter in the code first migration.

CreateTable(
    "dbo.SomeTable",
    c => new
        {
            TheDateField = c.DateTime(defaultValueSql: "GETDATE()")
        });

I don't like the often reference solution of setting it in the entity class constructor because if anything other than Entity Framework sticks a record in that table, the date field won't get a default value. And the idea of using a trigger to handle that case just seems wrong to me.