To change the start date of the gant chart, use the ribbon menu:
Or the left menu:
You may consider to just click on « Auto »: the macro will automatically look for the earliest date in your planning and set it as a starting date. From time to time, you can click on this icon to reflect the latest updates you make to your chart.
Another way is to click on « Today ». The Gantt Chart will use today date as a starting date.