dataStart

Specifies an explicit value for a data series start:

dataSeries: [
    {
        seriesType: 'splinearea',
        data: [29.9, 71.5, 106.4, 129.2, 144.0, 176.0, 135.6, 
        148.5, 216.4, 194.1, 95.6, 54.4],
        dataStart: Date.UTC(2010, 0, 1),
        dataStep: 24 * 3600 * 1000
    }
]

This setting is demonstrated in the following example.