function sundays(startYear, startMonth, startDay, endYear, endMonth, endDay) {
    var start = ;
    var end = ;
    return count(endYear, endMonth, endDay) - count(startYear, startMonth, startDay);
    
    function count(year, month, day) {
        var monthDays = [ 31, isLeap(year) ? 29 : 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 ];
        for (var days = 0, i = 1900; i < year; ++i) {
            days += isLeap(year) ? 366 : 365;
        }
        days += monthDays.slice(0, month-1).reduce(function(start, days) {
            return start + days;
        }, day);
        return days / 7 | 0;
    }
    
    function isLeap(year) {
        return (year % 4) && ((year % 100) > 0 || !(year % 400));
    }
}

sundays(1901, 1, 1, 2000, 12, 31);