How to calculate number of months between two dates using javascript

JavaScript
4 

var a = someDate;
var b = someOtherDate;

// Months between years.
var months = (b.getFullYear() - a.getFullYear()) * 12;

// Months between... months.
months += b.getMonth() - a.getMonth();

// Subtract one month if b's date is less that a's.
if (b.getDate() < a.getDate())
{
    months--;
}