How to convert API content field created and modified into datetime

I' trying to get list of users and pinboards from ThoughtSpot by calling API.

This is example output from get users API:

How to convert value of created and modified into datetime format?

[
  {
    "name": "Administrator",
    "displayName": "Administration Group",
    "created": 1354006445722,
    "modified": 1354006445987,
    "principalTypeEnum": "LOCAL_GROUP",
    "groupNames": [],
    "visibility": "DEFAULT"
  },
  {
    "name": "Analyst",
    "displayName": "Analyst Group",
    "created": 1354006445722,
    "modified": 1354006445987,
    "principalTypeEnum": "LOCAL_GROUP",
    "groupNames": [],
    "visibility": "DEFAULT"
  },
  {
    "name": "rls-group-3",
    "displayName": "rls-group-3",
    "description": "Contains directly rls-group-1, rls-group-2 and belongs direclty to rls-group-5",
    "created": 1459376495060,
    "modified": 1459376590681,
    "principalTypeEnum": "LOCAL_GROUP",
    "groupNames": ["rls-group-5"],
    "visibility": "DEFAULT"
  }
  ]
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