Returns a new CalculationDate relative to the CalculationDate on the same day of the week for the week previous to the one that contains the CalculationDate.
var date = now.valueOf('2017-03-15'); // a Wednesday
var lastWeek = date.lastWeek();
console.log('lastWeek = %s', lastWeek);
//lastWeek = 2017-02-08 00:00:00