You can easily change the start and due date of any task or check-in so long as HR hasn't locked that down.
The start date is the day the recipient will receive an email notifying them of the task and/or check-in question. The Due Date is when this activity must be completed by. Any activity left incomplete by the end of the Due Date will move to overdue status.
To change the dates, simply click on the task or check-in in question within the new hire's calendar. For tasks or check-ins that you as a manager must complete, you can also change the dates by clicking on the task or check-in within your own activity calendar. Simply click on either entry box and a calendar will appear allowing you to select any valid date to Start the task or mark it as Due. Don't forget to click the Save button or else your changes will not be recorded.
Note the section to the right called: When do you want to send this task and how long should it take?
[doesn't have info on how to change the start date]