public static Integer DayOfWeek(Integer day, Integer month, Integer year) {
     Double a = Math.floor((14 - month) / 12);
     Double y = year - a;
     Double m = month + 12 * a - 2;
     Double myDouble1 = day + y + Math.floor(y / 4) - Math.floor(y / 100) + Math.floor(y / 400) + Math.floor((31 * m) / 12);
     Integer myInt1 = myDouble1.intValue();
     Integer myInt2 = 7;
     Integer myInt3 = Math.mod(myInt1, myInt2);      
     return myInt3;
 }
 
 public static Double gregdaynumber(Integer year, Integer month, Integer day){
  // computes the day number since 0 January 0 CE (Gregorian)   
  Integer y=year;
  Integer m=month+1;
  if(month < 3) 
   y=y-1;
  if(month < 3) 
   m=m+12;
  return Math.floor(365.25*y)-Math.floor(y/100)+Math.floor(y/400)+Math.floor(30.6*m)+day-62;
 }
 
 public Double isoweeknumber(Date d1){
    Double isoweek;
   
    Integer year = d1.year();
    Integer month = d1.month() - 1;
    Integer day = d1.day();
    Integer wday = DayOfWeek(day, month, year);
    System.debug('the weekday is:' + wday);
   
    wday = Math.mod((wday+6),7) + 1; // weekdays will be numbered 1 to 7
   
    Integer yiso = year;
   
    Double d0 = gregdaynumber(year,1,0);
    Double wday0 = Math.mod((d0+4).intValue(),7) + 1;
   
    Double d = gregdaynumber(year,month+1,day);
    Double wniso = Math.floor((d-d0+wday0+6)/7)-Math.floor((wday0+3)/7);
   
  // check whether the last few days of December belong to the next year's ISO week
   
    if((month == 11) && ((day-wday) > 27)){
      wniso=1;
      yiso=yiso+1;
    }
   
  // check whether the first few days of January belong to the previous year's ISO week
   
    if((month == 0) && ((wday-day) > 3)){
      d0=gregdaynumber(year-1,1,0);
      wday0=Math.mod((d0+4).intValue(),7) + 1;
      wniso=Math.floor((d-d0+wday0+6)/7)-Math.floor((wday0+3)/7);
      yiso=yiso-1;
    }
   
    return wniso;
  }