I am trying to set up an End of Year formula that runs every year on December 31st. This would be a financial calculation that I would like to run. Any help, tips, or guidance would be greatly appreciated! 

I am trying to set up an End of Year formula that runs every year on December 31st. This would be a financial calculation that I would like to run. Any help, tips, or guidance would be greatly appreciated! 

Validation Rule: Length Must be 9 Digits, and 9 Digits only, (it is a text field). AND the value should also be able to save (not fire error message) if there is no value in the field (NULL). I have tried several attempts, and while testing the validation rule, I have not been able to get the Null Value part to work. Validation error message is firing for 9 Digits only, and be restricted to 9 digits only, but not null... Any help would be awesome!

OR(
     NOT(
         ISNUMBER(
                  Employer_ID_No_EIN__c
                 )
        ),
    AND(
        NOT(
            ISBLANK(
                     Employer_ID_No_EIN__c
                   )
           ),   
       
        
        ISNUMBER(Employer_ID_No_EIN__c), 
        LEN(Employer_ID_No_EIN__c) <>9
          
       )
)