C++ smart pointer affect memory leak issue

Question

I've write one simple C++ program to test the smart pointer, but I've found that the smart pointer affect the memory leak issue. Please see below code for it:

test.cpp: (have tested it in the CLANG 7.0 or gcc 7.3.0)

#include <bits/stdc++.h>

using namespace std;

class Date {
private:
  int day = 0, month = 0, year = 0;
  string dateInString;

public:
  Date() { cout << "Date Constructor:" << *this << endl; }

  Date(int inMonth, int inDay, int inYear)
    : day(inDay), month(inMonth), year(inYear)
  {
    cout << "Date Constructor: " << *this << ends;
    DisplayDate();
  }

  Date(const Date &copySource)
  {
    cout << "Date Copy Constructor." << *this << endl;
    day   = copySource.day;
    month = copySource.month;
    year  = copySource.year;
  }

  Date(Date &&moveSource)
  {
    cout << "Date Move Constructor." << *this << endl;
    day   = moveSource.day;
    month = moveSource.month;
    year  = moveSource.year;
  }

  virtual ~Date() { cout << "Date Destructor: " << *this << endl; }

  const Date &operator=(const Date &copySource)
  {
    cout << "Date Assignment Operator=" << *this << endl;
    if (this != &copySource) {
      day   = copySource.day;
      month = copySource.month;
      year  = copySource.year;
    }
    return *this;
  }

  operator const char *()
  {
    // assists string construction
    ostringstream formattedDate;
    formattedDate << month << "/" << day << "/" << year;
    dateInString = formattedDate.str();
    return dateInString.c_str();
  }

  void DisplayDate() { cout << month << "/" << day << "/" << year << endl; }

  // prefix increment
  Date &operator++()
  {
    ++day;
    return *this;
  }

  // prefix decrement
  Date &operator--()
  {
    --day;
    return *this;
  }

  // postfix increment
  Date operator++(int postfix)
  {
    Date copy(month, day, year);
    ++day;
    // copy of instance increment returned
    return copy;
  }

  // postfix decrement
  Date operator--(int postfix)
  {
    cout << "operator--" << endl;
    auto p = make_unique<Date>(month, day, year);
    // auto p = new Date(month, day, year);
    --day;
    // copy of instance increment returned
    return *p;
  }
};

auto main() -> decltype(0)
{
  Date holiday(12, 25, 2016);

  Date postfixDay1;
  postfixDay1 = holiday--;
  cout << "holiday after a postfix-decrement is : ";
  holiday.DisplayDate();

  cout << "postfixDay1 after a postfix-decrement is : ";
  postfixDay1.DisplayDate();

  return 0;
}

The program will output below message:

Date Constructor: 12/25/2016 12/25/2016
Date Constructor:0/0/0
operator--
Date Constructor: 12/25/2016 12/25/2016
Date Copy Constructor.0/0/0
Date Destructor: 12/25/2016
Date Assignment Operator=0/0/0
Date Destructor: 12/25/2016
holiday after a postfix-decrement is : 12/24/2016
postfixDay1 after a postfix-decrement is : 12/25/2016
Date Destructor: 12/25/2016
Date Destructor: 12/24/2016

Summary: We can found that there are three constructor invoked on, but there are four destructor invoked on! it means that the smart pointer has been destructored twice.

Why the smart pointer can't handle this case?