How do you convert Excel Date type to DateTime in .NET?

am using the Office Primary Interop Assemblies (automation) to access an
Excel file from .NET and get cell contents. I'm using C#.

When I get the cell contents that contains an Excel Date value, I am having
trouble converting from the object variable type to DateTime.
The following error is displayed:

               The specified cast is invalid.

Here's the code with error:

Range rng = (Range)oSheet.get_Range("A4", Type.Missing);
System.Object myvalue = rng.Cells.Value2; //Assign it to an object var
DateTime dt = (DateTime)myvalue;

So how do I cast the retrieved cell contents to .NET DateTime?