Using the IF statements will only give the month of the Start Date but you also need to flag the months between the Start and End Dates as a service month.
I have attach a sample database (2007 format) that uses code to populate an array to detemine the months of service (assuming the period of service is not more than 12 months). The array is then written to a table and a query is used to display the totals.
Run the macro macro_year_totals.
tbl_clients - table that has the raw data
tbl_services - table that stores the calculated data from the array
qry_year_services - a query that displays the total
macro_year_totals - runs the code and opens the query qry_year_services
mod_client_services - runs code to read each record in the raw data, flag each month between the start and finish dates as a month of service and store these values in an array. When all the records in the raw data are read the output is written to tbl_services.