error passing a marshal-by-refernce-class

[tolisss]

Hi i have the following class

public class User : XPBaseObject 
{
	public DateTime ReceiptTimeIntervalStart;
}

and i m trying to

Console.Write(user.ReceiptTimeIntervalStart.ToShortTimeString());

but the compiler says L:\NET Solutions\C#\OrderManager\frmUser.cs(200): Cannot pass 'OrderManager.User.ReceiptTimeIntervalStart' as ref or out, because 'OrderManager.User.ReceiptTimeIntervalStart' is a marshal-by-reference class

can someone help me please