setMonth()

var myDate = new Date()
// assume myDate = Wed Aug 1 2001
var currDate = myDate.setMonth(4)
var showDate = myDate
// showDate = Mon Apr 9 2001
var currDate = myDate.setDate(-13)
showDate = myDate
// showDate = Sun Jan 9 2000

Note:
The day of the week is automatically worked out by the system.