Why am I getting a marshal_by_reference warning

Question

Here is a simplified version of my code:

public partial class Master : Form
{
   public DateTime TradingPausedDT;
}

public class Other
{
   public Master masterForm;

   public void myFunc()
   {
         string fromDateTxt =      masterForm.TradingPausedDT.ToString("yyyyMMddTHHmmssfff");
   }
}

Here is the warning:

Accessing a member on "Master.TradingPausedDT" may cause an error because it is a field of a marshal-by-reference class.

Why am I getting this warning?