Sample Formula:
IF(
YEAR( End_Date__c ) = YEAR( Start_Date__c ) && MONTH( End_Date__c ) = MONTH( Start_Date__c ),
0,
IF ( YEAR( End_Date__c ) = YEAR( Start_Date__c ),
MONTH( End_Date__c ) – MONTH( Start_Date__c ) – 1,
( ( YEAR( End_Date__c ) – YEAR( TODAY() ) ) * 12 ) – ( 13 – MONTH( End_Date__c ) ) + ( 12 – MONTH( Start_Date__c ) )
)
)
YEAR( End_Date__c ) = YEAR( Start_Date__c ) && MONTH( End_Date__c ) = MONTH( Start_Date__c ),
0,
IF ( YEAR( End_Date__c ) = YEAR( Start_Date__c ),
MONTH( End_Date__c ) – MONTH( Start_Date__c ) – 1,
( ( YEAR( End_Date__c ) – YEAR( TODAY() ) ) * 12 ) – ( 13 – MONTH( End_Date__c ) ) + ( 12 – MONTH( Start_Date__c ) )
)
)
Output: