Resolved: using [Today] in formula of Calculated column.

Situation:
I have a check list is listing the domain name of customers who registered the domain with my services. I need to know the remaining of the validation date and send to customer an email. The expiration day is calculated from due date until today.

Problem:
I created a list with Due Date column with DateTime formated. Another column called remain days belongs to calculated with the formula like this: [Due Date] - [Today]. After click save, I got an error:

That means, I can not use the SP variables, but I can define a custom column with the formula had an existed SP variable such as [Today] and [Me]. And I have another solutions:

1) Create a new column named Today with type of DateTime
2) Modify the Remains column if you had created before. Change the formula to [Due Date] - [Today] with [Today] is a new column.
3) Delete [Today] column.

As a result, the Remains column is calculated from Due Date to today.