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## Class template date

boost::date_time::date — Representation of timepoint at the one day level resolution.

## Synopsis

    // In header: <boost/date_time/date.hpp>

    template<typename T, typename calendar, typename duration_type_>
    class date :
      private boost::less_than_comparable< T, boost::equality_comparable< T > >
    {
    public:
      // types
      typedef T                          date_type;
      typedef calendar                   calendar_type;
      typedef calendar::date_traits_type traits_type;
      typedef duration_type_             duration_type;
      typedef calendar::year_type        year_type;
      typedef calendar::month_type       month_type;
      typedef calendar::day_type         day_type;
      typedef calendar::ymd_type         ymd_type;
      typedef calendar::date_rep_type    date_rep_type;
      typedef calendar::date_int_type    date_int_type;
      typedef calendar::day_of_week_type day_of_week_type;

      // construct/copy/destruct
      date(year_type, month_type, day_type);
      date(const ymd_type &);
      explicit date(date_int_type);
      explicit date(date_rep_type);

      // public member functions
      year_type year() const;
      month_type month() const;
      day_type day() const;
      day_of_week_type day_of_week() const;
      ymd_type year_month_day() const;
      bool operator<(const date_type &) const;
      bool operator==(const date_type &) const;
      bool is_special() const;
      bool is_not_a_date() const;
      bool is_infinity() const;
      bool is_pos_infinity() const;
      bool is_neg_infinity() const;
      special_values as_special() const;
      duration_type operator-(const date_type &) const;
      date_type operator-(const duration_type &) const;
      date_type operator-=(const duration_type &);
      date_rep_type day_count() const;
      date_type operator+(const duration_type &) const;
      date_type operator+=(const duration_type &);
    };

## Description

The date template represents an interface shell for a date class that is based on a year-month-day system such as the gregorian or iso systems. It provides basic operations to enable calculation and comparisons.

Theory

This date representation fundamentally departs from the C tm struct approach. The goal for this type is to provide efficient date operations (add, subtract) and storage (minimize space to represent) in a concrete class. Thus, the date uses a count internally to represent a particular date. The calendar parameter defines the policies for converting the the year-month-day and internal counted form here. Applications that need to perform heavy formatting of the same date repeatedly will perform better by using the year-month-day representation.

Internally the date uses a day number to represent the date. This is a monotonic time representation. This representation allows for fast comparison as well as simplifying the creation of writing numeric operations. Essentially, the internal day number is like adjusted julian day. The adjustment is determined by the Epoch date which is represented as day 1 of the calendar. Day 0 is reserved for negative infinity so that any actual date is automatically greater than negative infinity. When a date is constructed from a date or formatted for output, the appropriate conversions are applied to create the year, month, day representations.

### `date` public construct/copy/destruct

1.     date(year_type y, month_type m, day_type d);

2.     date(const ymd_type & ymd);

3.     explicit date(date_int_type days);

   This is a private constructor which allows for the creation of new dates. It is not exposed to users since that would require class users to understand the inner workings of the date class.

4.     explicit date(date_rep_type days);

### `date` public member functions

1.     year_type year() const;

2.     month_type month() const;

3.     day_type day() const;

4.     day_of_week_type day_of_week() const;

5.     ymd_type year_month_day() const;

6.     bool operator<(const date_type & rhs) const;

7.     bool operator==(const date_type & rhs) const;

8.     bool is_special() const;

   check to see if date is a special value

9.     bool is_not_a_date() const;

   check to see if date is not a value

10.     bool is_infinity() const;

    check to see if date is one of the infinity values

11.     bool is_pos_infinity() const;

    check to see if date is greater than all possible dates

12.     bool is_neg_infinity() const;

    check to see if date is greater than all possible dates

13.     special_values as_special() const;

    return as a special value or a not_special if a normal date

14.     duration_type operator-(const date_type & d) const;

15.     date_type operator-(const duration_type & dd) const;

16.     date_type operator-=(const duration_type & dd);

17.     date_rep_type day_count() const;

18.     date_type operator+(const duration_type & dd) const;

19.     date_type operator+=(const duration_type & dd);