Action card date sync will allow subaction due dates to 'roll up' to the parent card. By default these two date fields are unlinked, but you can turn on this date sync in project settings.
Creating a new project
When creating a new project, action card date sync is available underneath the header labeled 'Action card date settings'.
Editing an existing project
In order to change this setting in an existing project, navigate to 'More' > Edit Project > Action card date settings.
With this setting turned on, the start and end date of your action card(s) will automatically be updated as your subactions' dates change.