[Solved]-highstock selected rangeSelector is computed from the current end date in the slider and not from the start date-Highcharts

You can use setExtremes method in chart's load event and set the initial selected area.

chart: {
  ...,
  events: {
    load: function() {
      const xAxis = this.xAxis[0];
      xAxis.setExtremes(
        xAxis.dataMin,
        xAxis.dataMin + (xAxis.max - xAxis.min),
        true,
        false
      );
    }
  }
}

Live demo: https://jsfiddle.net/BlackLabel/hm4xLzft/

API Reference: https://api.highcharts.com/class-reference/Highcharts.Axis#setExtremes