Formula Field Help: Count between two dates with IF condition

I need help with a Formula. This logic does not work for me.... IF(ISBLANK(FieldA__c),FieldB__c - Today(),FieldA__c - FieldB) What this says. If FieldA is blank. Subtract Field B from Today. If FieldA is Not Blank. Subtract FieldA from FeildB. Please Assist. Thank You