subtracting dates in javascript

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Javascript

hi people i have a function looks like this

var months = new Array(12);
months[0] = "JANUARY";
months[1] = "FEBRUARY";
months[2] = "MARCH";
months[3] = "APRIL";
months[4] = "MAY";
months[5] = "JUNE";
months[6] = "JULY";
months[7] = "AUGUST";
months[8] = "SEPTEMBER";
months[9] = "OCTOBER";
months[10] = "NOVEMBER";
months[11] = "DECEMBER";

var current_date=new Date();
month_value = current_date.getMonth();

Now i am trying to get this function to get the month of 7 days back

How do i do that?