Support timezone

[ASaiAnudeep]

{
  "schedule": {
    "layers": [
      {
       "start": "10:00:00",
       "tz": "GMT",
        "rotation": {
          "every": "week",
          "users": [
            {
              "name": "Jon",
              "slack_uid": "ULA15K66N"
            }
          ]
        }
      }
    ]
  }
}