TimeAxis, I still don't get how to handle it



I have a chart with DateTime xAxis 

I still don’t get how the date are stored internally and how to handle them.

My problem: I have a NSDate and I need to center a chart around this NSDate, keeping the current visible range span

I don’t see a working way to transform the NSDate and  the chart.xAxis.axisRange.span and chart.xAxis.axisRange.minimum in the same system so I can confront them and manage