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/*********************************************************************/
/* The following code calculates the start and end dates for         */
/* daylight saving time for the years 2006 through 2009.             */
/*                                                                   */
/* FDOY represents the first day of the year                         */
/* FDO_APR represents the first day of April                         */
/* DST_BEG represents the first day of daylight saving time          */
/*                                                                   */
/* It is calculated using this syntax:                               */
/*    intnx('week.1',fdo_apr,(weekday(fdo_apr) ne 1))                */
/*                                                                   */
/* Explanation of the above syntax:                                  */
/*   - weekday(fdo_apr) returns a number between 1 and 7,            */
/*     which represents the day of the week, 1=Sunday and 7=Saturday */
/*                                                                   */
/*   - weekday(fdo_apr) ne 1) returns a 0 if the first day           */
/*     of April is a Sunday and 1 otherwise.                         */
/*                                                                   */
/*   - intnx('week.1',fdo_apr,(weekday(fdo_apr) ne 1)) returns a     */
/*     SAS date which is 0 WEEK intervals from the FDO_APR, if the   */
/*     first day of April is a Sunday. The WEEK.1 interval specifies */ 
/*     Sunday as the first day of the week.                          */
/*                                                                   */
/*   OR                                                              */
/*                                                                   */
/*   returns a SAS date which is 1 WEEK interval from the FDO_APR,   */
/*   if the first day of April is not a Sunday, (using Sunday as     */
/*   the first day of the week).                                     */
/*                                                                   */
/*                                                                   */
/* DST_END represents the end of daylight saving time and is         */
/* calculated similarly to DST_BEG.                                  */
/*********************************************************************/


data _null_;
  do year=2006 to 2009;
    fdoy=mdy(1,1,year);
    /* For years prior to 2007, daylight time begins in the United States on */
    /* the first Sunday in April and ends on the last Sunday in October.     */
    if year <= 2006 then do;
      fdo_apr=intnx('month',fdoy,3);
      dst_beg=intnx('week.1',fdo_apr,(weekday(fdo_apr) ne 1));
      fdo_oct=intnx('month',fdoy,9);
      dst_end=intnx('week.1',fdo_oct,(weekday(fdo_oct) in (6,7))+4);
    end;

    /* Due to the Energy Policy Act of 2005, Pub. L. no. 109-58, 119 Stat 594 */
    /* (2005).  Starting in March 2007, daylight time in the United States    */
    /* will begin on the second Sunday in March and end on the first Sunday   */
    /* in November.  For more information, one reference is                   */
    /* http://aa.usno.navy.mil                                                */
    else do;  
      fdo_mar=intnx('month',fdoy,2);
      dst_beg=intnx('week.1',fdo_mar, (weekday(fdo_mar) in (2,3,4,5,6,7))+1);
      fdo_nov=intnx('month',fdoy,10);
      dst_end=intnx('week.1',fdo_nov,(weekday(fdo_nov) ne 1));
    end;
    put dst_beg= worddate. /
    dst_end= worddate. / ;
  end;
run;

OUTPUT to SAS log 


dst_beg=April 2, 2006
dst_end=October 29, 2006

dst_beg=March 11, 2007
dst_end=November 4, 2007

dst_beg=March 9, 2008
dst_end=November 2, 2008

dst_beg=March 8, 2009
dst_end=November 1, 2009

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