Returns a new DateRange relative to the CalculationDate starting nine days before the CalculationDate up to and including the CalculationDate.
var date = now.valueOf('2017-03-15');
var range = date.last10days();
console.log('Date range = %s', range);
//Date range = 2017-03-06 00:00:00 -> 2017-03-15 00:00:00