Calculation fields within REDCAp instruments, by month

Question

Is there a way to get the calculation field in REDCAp to sum the responses by month instead of a cumulative of all times the form was filled out?

Ex. I have

sum([root][cumulative_variable][previous-instance],[root][variable][current-instance])

which gives me a summary of the values TO DATE. There is also a date field on the same form which takes 1=Jan, 2=Feb, etc. Is there a way to use if/then logic to ask the field to display only the cumulative total of the current month?

My alternative to doing this is creating a separate event for each month instead of a repeatable instrument. I want to avoid this because of the complexity of importing it into SQL server later.