How can I cast an extracted variable type timestamp to type date?

This kind of Javascript should convert a timestamp to a date format:

var myvalue = context.variableManager.getValue("Extracted");// Extracted is your variable extractor name
var mydate = new Date(0);
mydate.setUTCSeconds(myvalue);
logger.debug("new format="+mydate);

You may need to customize it so it can suit your need.