Js calculates the instance code for the days difference between two times

//Determine if it is a leap year
function isLeapYear(year){
if(year % 4 == 0 && ((year % 100 != 0) || (year % 400 == 0)))
{
     return true;
}
return false;
}
//Determine the before and after dates
function validatePeriod(fyear,fmonth,fday,byear,bmonth,bday){
if(fyear < byear){
return true;
}else if(fyear == byear){
if(fmonth < bmonth){
   return true;
} else if (fmonth == bmonth){
   if(fday <= bday){
    return true;
   }else {
    return false;
   }
} else {
   return false;
}
}else {
return false;
}
}
//Calculate the difference between the two dates
function dateDiff(d1,d2){
    var disNum=compareDate(d1,d2);
    return disNum;
}
function compareDate(date1,date2)
{
    var regexp=/^(d{1,4})[-|.]{1}(d{1,2})[-|.]{1}(d{1,2})$/;
    var monthDays=[0,3,0,1,0,1,0,0,1,0,0,1];
    regexp.test(date1);
    var date1Year=RegExp.$1;
    var date1Month=RegExp.$2;
    var date1Day=RegExp.$3;
    regexp.test(date2);
    var date2Year=RegExp.$1;
    var date2Month=RegExp.$2;
    var date2Day=RegExp.$3;
if(validatePeriod(date1Year,date1Month,date1Day,date2Year,date2Month,date2Day)){
firstDate=new Date(date1Year,date1Month,date1Day);
     secondDate=new Date(date2Year,date2Month,date2Day);
     result=Math.floor((secondDate.getTime()-firstDate.getTime())/(1000*3600*24));
     for(j=date1Year;j<=date2Year;j++){
  if(isLeapYear(j)){
      monthDays[1]=2;
  }else{
      monthDays[1]=3;
  }
  for(i=date1Month-1;i<date2Month;i++){
      result=result-monthDays[i];
  }
     }
     return result;
}else{
    alert(' Sorry, the first time must be less than the second time, thank you! ');
    exit;
}
}

Calling this function passes two values of time: 2013-01-19& PI;   2013-12-19

Days = dateDiff (d1, d2);