C++ != C#. :)

In C++, you must declare d a pointer:

  Day* d = new Day;
  d->whatever();

But unless you have a reason to create it dynamically, you're better off
declaring it as below:

  Day d;
  d.whatever();

This relieves you from worrying about the lifetime of the Day object, which
you must think about given that Day is a non-GC class per your error
message. The second form ensures that d is destroyed when it goes out of
scope.