Okay so now you've created the class Date you need to create an instance of it.
This is like saying you've created the concept of a Date, now you need to make a Date.
This is actually simpler than it sounds. To create a date you simply write Date name_of_date;
You can then call ++name_of_date;
to increment the date. However as you have no constructor or 'get' and 'set' functions there's no knowing what name_of_date's values will be.
And in answer to your final question yes, though you may need to write int unused in the definition