convert time stamp to Date inside angular expression like {{new Date(timestamp)}}

Because angular expressions do not accept arbitrary Javascript code, including creating new objects. What you are looking for is called angular filters, they were specifically designed for this case.

So instead of

{{new Date(elem.timestamp)}}

you should write

{{elem.timestamp | date: 'yyyy-MM-dd'}}

More info on date filter can be found here. More on filters in general can be found here.

If you are dealing with Firebase Timestamp object (https://firebase.google.com/docs/reference/android/com/google/firebase/Timestamp), you can use:

{{elem.timestamp.seconds*1000 | date}}

Just an extention of what @package mentioned we can also go to hour-second as well from millisecond, the format can be any pattern as well

< th >{{elem.timestamp | date: 'yyyy/MM/dd h:mm:ss a'}}< /th >