How do we specify default value only for an Update?

I have the following dto:

public class File
    public Guid Id { get; set; }
    public string Name { get; set; }
    public DateTime Created { get; set; }
    public DateTime? Updated { get; set; }

I want Created to be set only on Insert (this is working) and Updated to be only set on Updates (this isn’t working). Using PostgreSQL dialect.

The default values are only when fields are excluded during INSERT’s, there are no excluded defaults in UPDATE statements, they fields are just not updated.