date.js - get monday of given week by date

Question

How to get a Monday of given week based on supplied date?

When I do BaseDate.parse("last monday"); I am getting an error.

InitDate = "29/10/2013" // Some Date
BaseDate = Date.parse(InitDate); // Tue Oct 28 2013 00:00:00 GMT+0000 (GMT Standard Time)  

CurrentWeekMonday = BaseDate.parse("last monday");
//TypeError: Object Wed Nov 28 2012 00:00:00 GMT+0000 (GMT Standard Time) has no method 'parse' 

Any suggestion much appreciated.