IF(
AND(
YEAR(End_Date__c) = YEAR(Begin_Date__c),
MONTH(End_Date__c) = MONTH(Begin_Date__c)),
1,
IF(ISBLANK(End_Date__c),
(YEAR(TODAY())*12+MONTH(TODAY())) –
(YEAR(Begin_Date__c)*12+MONTH(Begin_Date__c)),
(YEAR(End_Date__c)*12+MONTH(End_Date__c)) –
(YEAR(Begin_Date__c)*12+MONTH(Begin_Date__c))
)
)