As you are inputting tasks for your Sequence, you will be required to put in "Due Date from Start" for each task. This simply means how many days from the start of your Sequence you want the task due.
Since task due dates are dynamic from the start of the Sequence, you will be required to put in a number of days instead of a strict date. Sequence will dynamically let you know what day your task is due and if a task is overdue.