Extract Only Date from DateTime Object in Javascript?

Question

i have Json Array which look like this and its working fine but i need to arrange them gruopwise like DateWise

 [ 
name: 'Ali',
startedAt: Wed Dec 28 2016 15:32:07 GMT+0500 (Pakistan Standard Time),
userId: '0002' },

{ completedAt: Wed Dec 28 2016 12:51:47 GMT+0500 (Pakistan Standard Time),
name: 'Aliy',
startedAt: Wed Dec 28 2016 12:43:19 GMT+0500 (Pakistan Standard Time),
userId: '212' },
{
name: 'Aliy',
startedAt: Wed Dec 28 2016 12:43:06 GMT+0500 (Pakistan Standard Time),
userId: '2121' }]

i have a code which groups them on the basis of startedAt and its working fine but the problem is that i want only date part like 28/12/2016

the below code is used for grouping

var groupedData = _.groupBy(datas, function(d){
   return d.startedAt});}