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
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