Set Date/Time to 00:00:00

Google Apps Script is JavaScript, date manipulations are explained here,

setting the hours goes like this :

Date.setHours(hour,min,sec,millisec)

here is an example :

//   3/16/2013 17:00:00

function test(){
 var x = new Date('3/16/2013 17:00:00');// x is now a date object
 x.setHours(0,0,0,0); set  hours to 0, min Secs and milliSecs as well
 Logger.log(x);// show result
}

Logger value : Sat Mar 16 00:00:00 GMT+01:00 2013