Annual employee salary increase - after one full year of service -- formula

Hello,

My brain has apparently taken a logic holiday. I'm trying to create a formula that gives a specific employee a raise after 12 months of service. For a given position (e.g., "Position A" in the attached example), the headcount will incrementally increase, so I want to be able to evaluate when the headcount increased 13 months prior to the current month and then apply the salary increase (5%, in the attached example).

Thank you for any thoughts and/or help!