(javascript or/and datejs)Trying to calculate two dates intervals total salary. when start date and end date has some fraction days of month

****Salary type monthly.****

var getDiffDatesSalary = function days_between(date1,date2, monthlyBasic) {
        var dateObj1 = new Date(date1);
        var month1 = dateObj1.getMonth(); //months from 1-12
        var day1 = dateObj1.getDate();
        var year1 = dateObj1.getFullYear();
        var daysInMonth1 = Date.getDaysInMonth(year1, month1);

        var dateObj2 = new Date(date2);
        var month2 = dateObj2.getMonth(); //months from 0-11
        var day2 = dateObj2.getDate();// days from 1
        var year2 = dateObj2.getFullYear();
        var daysInMonth2 = Date.getDaysInMonth(year2, month2);

        var date1FractionDays = daysInMonth1 - day1;
        var date2FractionDays = daysInMonth2 - day2;
        var newDate1, newDate2;
        if(day1 > 1){
            var perDaySalary1 = monthlyBasic / daysInMonth1;
            var thisMonthFarctionDaysSalaryForDate1 = perDaySalary1 * date1FractionDays;
            month1 += 1; // after calculate fraction this month basic, round month from next
            newDate1 = new Date(year1,month1);
        }
        if(day2 !== daysInMonth2){
           var perDaySalary2 = monthlyBasic / daysInMonth2;
           var thisMonthFarctionDaysSalaryForDate2 = perDaySalary2 * day2;
            month2 -= 1; //after calculate fraction this month basic, round month from previous
            newDate2 = new Date(year2,month2);
        }

       // i want to calculate totalSalaryamount of date ranges
        // var totalFractionDaysSalary = thisMonthFarctionDaysSalaryForDate1 + thisMonthFarctionDaysSalaryForDate2;
        // var totalMonthsSalay = roundMonths * monthlyBasic;
        // var totalSalaryamount = totalFractionDaysSalary + totalMonthsSalay;

    };

**

result = thisMonthFarctionDaysSalaryForDate1 + thisMonthFarctionDaysSalaryForDate2 + (roundMonths * monthlyBasic)

result will be getDiffDatesSalary("2016/01/15","2016/03/25", 1000);

516.13+806.45 + 1000 // total: 2322.58

Or/And getDiffDatesSalary("2015/01/15","2016/03/25", 1000) ;

516.13+806.45 + (1000 * 13) // total: 14322.58

Or/And getDiffDatesSalary("2016/01/01","2016/02/29", 1000);

1000* 2 // 2000

**

I suggest to split the task in two sections:

1. Get total days
2. Calculate salary.

This proposal uses the difference between years, months and days and makes a correction if the day difference is negative. Same day means one day.

function getDays(date1, date2) {
    var date1Obj = new Date(date1),
        date2Obj = new Date(date2),
        totalYear = date2Obj.getFullYear() - date1Obj.getFullYear(),
        totalMonth = date2Obj.getMonth() - date1Obj.getMonth(),
        totalDay = date2Obj.getDate() - date1Obj.getDate() + 1;

    if (totalDay < 0) {
        totalMonth--;
        totalDay += new Date(date1Obj.getFullYear(), date1Obj.getMonth(), 0).getDate();
    }

    return 360 * totalYear + 30 * totalMonth + totalDay;
}

function getDiffDatesSalary(date1, date2, salaryPerMonth) {
    return getDays(date1, date2) * salaryPerMonth / 30;
}

document.write(getDiffDatesSalary("2016/01/15", "2016/03/25", 1000) + '<br>');
document.write(getDiffDatesSalary("2016/01/31", "2016/02/15", 1000) + '<br>');
document.write(getDiffDatesSalary("2016/03/20", "2016/03/20", 3000) + '<br>');