[amibroker] Relating BarIndex with Calendar Date

[tipequity]

I am having a hard time relating dates to bar indecies. In code 
provided below I am trying to calculate the bar index of a year ago 
date with little success. I appreciate any help and explanation.

SetBarsRequired(10000);

function RefDays( Array, Days ) 
{ 
  td = DaysSince1900(); 
  result = Null; 

  if( Days < 0 ) 
  { 
     for( i = BarCount -1; i >= -Days; i = i - 1 ) 
     { 
       backday = td[ i ] + Days; // Days is negative 
       for( j = -Days/2; j < i; j++ ) 
       { 
          if( td[ i - j ] <= backday ) 
           { 
             result[ i ] = Array[ i - j ]; 
             break; 
           } 
       } 
     } 
  } 
  return result; 
} 

yy = Year();
dy = DayOfYear();
leapyear = ( yy % 4 ) == 0 AND yy != 2000;
yearlen = IIf( leapyear, -366, -365 ); 

aYearAgoDate = RefDays(DateTime(), LastValue(yearlen));
dn = DateNum();
BarIndx = BarIndex();

StartDateBar = LastValue( ValueWhen ( dn == StartDate ,BarIndx ) );