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## Struct template us_dst_trait

boost::date_time::us_dst_trait — Specification for daylight savings start rules in US.

## Synopsis

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

    template<typename date_type>
    struct us_dst_trait {
      // types
      typedef date_type::day_of_week_type                 day_of_week_type;
      typedef date_type::month_type                       month_type;
      typedef date_type::year_type                        year_type;
      typedef date_time::nth_kday_of_month< date_type >   start_rule_functor;
      typedef date_time::first_kday_of_month< date_type > end_rule_functor;
      typedef date_time::first_kday_of_month< date_type > start_rule_functor_pre2007;
      typedef date_time::last_kday_of_month< date_type >  end_rule_functor_pre2007;

      // public static functions
      static day_of_week_type start_day(year_type);
      static month_type start_month(year_type);
      static day_of_week_type end_day(year_type);
      static month_type end_month(year_type);
      static date_type local_dst_start_day(year_type);
      static date_type local_dst_end_day(year_type);
      static int dst_start_offset_minutes();
      static int dst_end_offset_minutes();
      static int dst_shift_length_minutes();
    };

## Description

This class is used to configure dst_calc_engine template typically as follows:

    using namespace boost::gregorian;
    using namespace boost::posix_time;
    typedef us_dst_trait<date> us_dst_traits;
    typedef boost::date_time::dst_calc_engine<date, time_duration,
                                              us_dst_traits>
                                              us_dst_calc;
    //calculate the 2002 transition day of USA April 7 2002
    date dst_start = us_dst_calc::local_dst_start_day(2002);

    //calculate the 2002 transition day of USA Oct 27 2002
    date dst_end = us_dst_calc::local_dst_end_day(2002);

    //check if a local time is in dst or not -- posible answers
    //are yes, no, invalid time label, ambiguous
    ptime t(...some time...);
    if (us_dst::local_is_dst(t.date(), t.time_of_day())
        == boost::date_time::is_not_in_dst)
    {

    }

This generates a type suitable for the calculation of dst transitions for the United States. Of course other templates can be used for other locales.

### `us_dst_trait` public static functions

1.     static day_of_week_type start_day(year_type);

2.     static month_type start_month(year_type y);

3.     static day_of_week_type end_day(year_type);

4.     static month_type end_month(year_type y);

5.     static date_type local_dst_start_day(year_type year);

6.     static date_type local_dst_end_day(year_type year);

7.     static int dst_start_offset_minutes();

8.     static int dst_end_offset_minutes();

9.     static int dst_shift_length_minutes();