This will get tomorrow's date:

var a = new Date((new Date()).valueOf() + 1000*3600*24);

You have to use the getDate() and setDate() methods of the Date object which respectively get and set the day value of the date.

var date = new Date();
date.setDate(date.getDate() + 1);

Check the MDC Date object reference for more information on working with dates

Try this:

//create the date
var myDate = new Date();

//add a day to the date
myDate.setDate(myDate.getDate() + 1);