If you just want to get the first day of the next month, you could do something like:
var now = new Date();
var future = now.setMonth(now.getMonth() + 1, 1);
var past = now.setMonth(now.getMonth() - 1, 1);
This will prevent the "next" month from skipping a month (e.g. adding a month to January 31, 2014 will result in March 3rd, 2014 if you omit the second parameter).
As an aside, using date.js* you could do the following:
var today = Date.today();
var past = Date.today().add(-1).months();
var future = Date.today().add(1).months();
In this example I am using today's date, but it works for any date.
*date.js has been abandoned. If you decide to use a library, you should probably use moment.js as RGraham suggests.