How do I add or edit the grid start date in the Gantt chart?

This will highlight where you can navigate to in the Gantt Chart to add or edit the Grid Start Date.

Within the Gantt view, you will now have an option to set the Grid Start Date.

The Grid Start Date will determine the start date for all items unless they have a constraint date specifying otherwise. 

This option is available on the top left-hand side within the Gantt view. 

To learn more on the Grid Start date and understand how you can add and edit it, please watch this video: