Matt Kruse developed a really interesting date library which should help with your particular case.
Here's a snippet of the method you should use for the issue you mentioned:
// ------------------------------------------------------------------
// parseDate( date_string [, prefer_euro_format] )
//
// This function takes a date string and tries to match it to a
// number of possible date formats to get the value. It will try to
// match against the following international formats, in this order:
// y-M-d MMM d, y MMM d,y y-MMM-d d-MMM-y MMM d
// M/d/y M-d-y M.d.y MMM-d M/d M-d
// d/M/y d-M-y d.M.y d-MMM d/M d-M
// A second argument may be passed to instruct the method to search
// for formats like d/M/y (european format) before M/d/y (American).
// Returns a Date object or null if no patterns match.
// ------------------------------------------------------------------
function parseDate(val) {
var preferEuro=(arguments.length==2)?arguments[1]:false;
generalFormats=new Array('y-M-d','MMM d, y','MMM d,y','y-MMM-d','d-MMM-y','MMM d');
monthFirst=new Array('M/d/y','M-d-y','M.d.y','MMM-d','M/d','M-d');
dateFirst =new Array('d/M/y','d-M-y','d.M.y','d-MMM','d/M','d-M');
var checkList=new Array('generalFormats',preferEuro?'dateFirst':'monthFirst',preferEuro?'monthFirst':'dateFirst');
var d=null;
for (var i=0; i<checkList.length; i++) {
var l=window[checkList[i]];
for (var j=0; j<l.length; j++) {
d=getDateFromFormat(val,l[j]);
if (d!=0) { return new Date(d); }
}
}
return null;
}