convert date to UTC ISO midnight

trying to create new date and set it to UTC ISO midnight and get output as following "2017-07-12T00:00:00" but no luck so far.

var dateNow = new Date();
var dateToUTC = dateNow.setUTCHours(0,0,0,0);
/*this gives me an error msg*/
var dateToISO = dateToUTC.toISOString();