How to get the date and time when uploading image to firebase?

You can use ServerValue.TIMESTAMP provided by the Firebase database server and
store it in a new timestamp field in your database along with the url and
description fields for each image. The Firebase servers write the current
timestamp in UTC milliseconds in place of this value when performing writes. 
This value is a 13-digit numeric value.

 mDatabaseRef.child(uploadID).child("timestamp").setValue(ServerValue.TIMESTAMP);