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C ++: polymorphism pattern template

Consider a class calendar that stores a bunch of Date objects. A calendar is designed to store a collection of objects of any type that inherit from Date. I thought the best way to do this is to create a class template like

template<typename D> class Calendar{ 
    ...
}

But it seemed to me that D can now be any class. Now my question is: how can I make sure that D is a subclass of the date object?

I know how to do this, this is Java, but I'm still not familiar with the C ++ syntax. The problem is very similar to how some collections can only accept template variables that implement Comparable. Then the headline would look like

public class Calendar<D extends Date>{
     ...
}

-------------------- EDIT: ------------------------- --- --------------

, . . , Calendar<Gregorian>, Date, , . . , Calendar<Gregorian>, Calendar<Julian>. :

Calendar<Gregorian> cal;
std::cout << "These events are entered as dates in 
    the Gregorian calendar" << std::endl;
cal.add_event("Christmas", 12, 25);
cal.add_event("Gregorian new year", 1, 1);
std::cout << cal << std::endl;
std::cout << "----" << std::endl;
std::cout << "And printed out as Julian dates" << std::endl;
Calendar<Julian>(cal);
std::cout << cal<< std::endl;

:

These events are entered as dates in the Gregorian calendar
2009-12-25 Christmas
2010-01-01 Gregorian new year
----
And printed out as Julian dates
2009-12-13 Christmas
2009-12-19 Gregorian new year

------------- : ----------------------

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     ...
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    ...
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