How to change default validation error message in ASP.NET MVC?

Say I have this property in my model:

[DisplayName("test")]
[Required(ErrorMessage = "required")]
public DateTime? SomeDate { get; set; }

when you type in "asdf" in Html.TextBoxFor(model => model.SomeDate), you get the validation error message "The value 'asdf' is not valid for test.".

How do you modify that message? ASP.NET MVC ignored [DataType(DataType.DateTime, ErrorMessage = 'some other message')]