Make sure that this is a field of type calculation, not a number field with an auto-enter calculation. If you set it up as an auto-enter calc, just change it to type calculation and you'll find that your calculation is still in place in this new field type.
2) Double click the field in Manage | Database | Fields to open the specify calculation dialog. Click the Storage Options button. Select that the calculation be an unstored calculation field and now it will update automatically with each new day.
Note, changing the field to unstored is necessary, but it may affect performance of any sorts and searches on this field and could prevent relationships that match to this field from working.