function checkValidity() {
  var form = document.forms[0];
  var now = new Date();
  var today = new Date(now.getYear(),now.getMonth(),now.getDate());
  var dateString = form.AvailableDate1.value;
  if (dateString=='') return;
  if (isValidDate(dateString,'/'))
      ;
  else {  
      alert ('Please enter a valid Availability date (dd/mm/yyyy) !  (' + reason + ')');
      form.AvailableDate1.focus();
      }
  }

function isValidDate (myDate,sep) {
// checks if date passed is in valid dd/mm/yyyy format

    if (myDate.length == 10) {
        if (myDate.substring(2,3) == sep && myDate.substring(5,6) == sep) {
            var date  = myDate.substring(0,2);
            var month = myDate.substring(3,5);
            var year  = myDate.substring(6,10);

            var test = new Date(year,month-1,date);

            if (year == y2k(test.getYear()) && (month-1 == test.getMonth()) && (date == test.getDate())) {
                reason = '';
                return true;
            }
            else {
                reason = 'valid format but an invalid date';
                return false;
            }
        }
        else {
            reason = 'invalid separators, must be ' && sep;
            return false;
        }
    }
    else {
        reason = 'invalid format';
        return false;
    }
}

function y2k(number) { return (number < 1000) ? number + 1900 : number; }