How to increment and decrement week?

How to increment and decrement week?

I have buttons for incrementing and decrementing weeks. I have to display
the start date and end date of the current week. When I click increment
button, start date and end date of the next week has to be displayed. If I
again click next start dat and end date of the week after that has to be
displayed. Similarly for decrement button.
var i = 0;
var curr = new Date; // get current date
var first = curr.getDate() - curr.getDay(); // First day is the day of
the month - the day of the week
var last = first + 6; // last day is the first day + 6
$('#btnPrevWeek,#btnNextWeek').click(function () {
if ($(this).is('#btnNext')) {
first = first + 7;
last = last + 7;
var firstday = new Date(curr.setDate(first)).toUTCString();
var lastday = new Date(curr.setDate(last)).toUTCString();
var startDatePieces = firstday.split(/[\s,]+/);
var endDatePieces = lastday.split(/[\s,]+/);
var startDate = startDatePieces[2] + " " + startDatePieces[1]
+ " " + startDatePieces[3];
var endDate = endDatePieces[2] + " " + endDatePieces[1] + " "
+ endDatePieces[3];
$('#lblWeekStartDate').html(startDate);
$('#lblWeekEndDate').html(endDate);
}
else {
first = first - 7;
last = last - 7;
var firstday = new Date(curr.setDate(first)).toUTCString();
var lastday = new Date(curr.setDate(last)).toUTCString();
var startDatePieces = firstday.split(/[\s,]+/);
var endDatePieces = lastday.split(/[\s,]+/);
var startDate = startDatePieces[2] + " " + startDatePieces[1]
+ " " + startDatePieces[3];
var endDate = endDatePieces[2] + " " + endDatePieces[1] + " "
+ endDatePieces[3];
$('#lblWeekStartDate').html(startDate);
$('#lblWeekEndDate').html(endDate);
}
})
I will be displaying the dates in this format - Sep 15 2013-Sep 21 2013 .
For the current month the code is working fine, after that it is not
working properly. Plese help me to fix the issue.