Carb Manager actually doesn't utilize the concept of a "start date".
If you've been away from your diet and are ready to start fresh, you can either change your Starting Weight, or completely reset your account.
Change Starting Weight
If you want to change your Starting Weight, you can go in and delete or modify old weight entries. To do so, navigate to My Measurements using the main (left) navigation menu.
Then, select the Weight tab.
Finally, scroll down to find your logged weight entries. Use the More Options (⋮) button to edit or delete your old logged entries as you see fit.