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## Struct template us_dst_trait

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

## Synopsis

    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 y) ;

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() ;

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