[EpiData-list] Manager /Calculate

Hello, In Manager, I do not understand the calculation of variables. I have a file with "birth date" and "date delivery" and "age" for the difference. and "weight" "size" and "BMI". When I click on the variable "Age" / edit / calculate, I can not find the variable "Age" for calculate the difference between two dates, but I see "BMI". On the other hand, on the variable "BMI" / edit / calculate, I do not find "BMI" but "Age" ..... Thank you for helping me.