see attached example created from the QDateTimeAxis example project.
Just added a second QLineSeries and created a QAreaSeries from both QLineSeries.
Then if I attached only the QAreaSeries series into the chart the xAxis minimum date is badly setup on 01.01.1970 whatever the data.
I put 2 alternative commented scenario where I put both the initial QLineSeries from the example and the QAreaSeries into the chart.
When the QLineSeries is put first, xAxis minimum date is again incorrect.
When the QAreaSeries is put first, xAxis minimum date is correct and follow the data