Returns a new CalculationDate relative to the CalculationDate shifted so that the time of day is 00:00:00. 

var date = now.valueOf('2017-03-15 15:00:00'); // 3pm on a Wednesday

var date = date.startOfDay();

console.log('date = %s', date);

//date = 2017-03-15 00:00:00
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