How can I deduce a day for a date object in javascript?

Question

How can I deduce a day for a date object in javascript?

Example:

If I assig a date into an object as below: var date_value = this.cfg.getProperty(cfgPageDate);

and the date format in date_value as below: Fri May 29 2009 00:00:00 GMT+1000 (AUS Eastern Standard Time)

how can I deduce a day from this object so that the object date will become as below:
Thu May 28 2009 00:00:00 GMT+1000 (AUS Eastern Standard Time)

Answer 1

var dat = new Date('2009/05/29');
alert (dat.setDate(dat.getDate()-1));