function calcBusinessDays(dDate1, dDate2) {         // input given as Date objects

  var iWeeks, iDateDiff, iAdjust = 0;

  if (dDate2 < dDate1) return -1;                 // error code if dates transposed

  var iWeekday1 = dDate1.getDay();                // day of week
  var iWeekday2 = dDate2.getDay();

  iWeekday1 = (iWeekday1 == 0) ? 7 : iWeekday1;   // change Sunday from 0 to 7
  iWeekday2 = (iWeekday2 == 0) ? 7 : iWeekday2;

  if ((iWeekday1 > 5) && (iWeekday2 > 5)) iAdjust = 1;  // adjustment if both days on weekend

  iWeekday1 = (iWeekday1 > 5) ? 5 : iWeekday1;    // only count weekdays
  iWeekday2 = (iWeekday2 > 5) ? 5 : iWeekday2;

  // calculate differnece in weeks (1000mS * 60sec * 60min * 24hrs * 7 days = 604800000)
  iWeeks = Math.floor((dDate2.getTime() - dDate1.getTime()) / 604800000)

  if (iWeekday1 <= iWeekday2) {
    iDateDiff = (iWeeks * 5) + (iWeekday2 - iWeekday1)
  } else {
    iDateDiff = ((iWeeks + 1) * 5) - (iWeekday1 - iWeekday2)
  }

  iDateDiff -= iAdjust                            // take into account both days on weekend

  return (iDateDiff + 1);                         // add 1 because dates are inclusive

}

function calcBusinessDays(dDate1, dDate2) {         // input given as Date objects

  var iWeeks, iDateDiff, iAdjust = 0;

  if (dDate2 < dDate1) return -1;                 // error code if dates transposed

  var iWeekday1 = dDate1.getDay();                // day of week
  var iWeekday2 = dDate2.getDay();

  iWeekday1 = (iWeekday1 == 0) ? 7 : iWeekday1;   // change Sunday from 0 to 7
  iWeekday2 = (iWeekday2 == 0) ? 7 : iWeekday2;

  if ((iWeekday1 + iWeekday2) > 11) iAdjust = 1;  // adjustment if both days on weekend

  iWeekday1 = (iWeekday1 > 5) ? 5 : iWeekday1;    // only count weekdays
  iWeekday2 = (iWeekday2 > 5) ? 5 : iWeekday2;

  // calculate differnece in weeks (1000mS * 60sec * 60min * 24hrs * 7 days = 604800000)
  iWeeks = Math.floor((dDate2.getTime() - dDate1.getTime()) / 604800000)

  if (iWeekday1 <= iWeekday2) {
    iDateDiff = (iWeeks * 5) + (iWeekday2 - iWeekday1)
  } else {
    iDateDiff = ((iWeeks + 1) * 5) - (iWeekday1 - iWeekday2)
  }

  iDateDiff -= iAdjust                            // take into account both days on weekend

  return (iDateDiff + 1);                         // add 1 because dates are inclusive

}

This routine is loosely based on elightbo's "Calculate Business Days" snippet in ASP, with some process & logic modifications (most notably accounting for situations when both dates fall on a weekend) and a translation to JavaScript.

Calculate Business Days between two dates

date, Business

JavaScript