JED (1.5.x and below) - Custom day cycle for the Overworld

{
    "dimensions": [
        {
            "dim": 0,
            "override": true,
            "dimensiontype": {
                "id": 0,
                "name": "Overworld",
                "suffix": "",
                "keeploaded": true,
                "worldprovider": "WorldProviderSurfaceJED"
            },
            "worldinfo": {
                "JED": {
                    "CustomDayCycle": true,
                    "DayLength": 1200,
                    "NightLength": 400
                }
            }
        }
    ]
}