There is no date time format function for javascript not using external libs. You have to do it yourself.
Here an examples for french format YYYY-mm-dd_HH:mm:ss

var curr = new Date();
var year = curr.getFullYear();
var month = (((curr.getMonth()+1) < 10)?'0:'') + (curr.getMonth()+1)
var day = ((curr.getDate() < 10)?'0':'')+curr.getDate()
var hour = ((curr.getHours() < 10)?'0':'')+curr.getHours()
var minute = ((curr.getMinutes() < 10)?'0':'')+curr.getMinutes()
var seconde = ((curr.getSeconds() < 10)?'0':'')+curr.getSeconds()

var output = year+'-'+month+'-'+day+'_'+hour+':'+minute+':'+seconde;