Calculate months of project participation within this year
I know how to calculate the months that a client is participating in our project (since the moment he signed up till the current date). Now I am wondering what the formula is to calculate the months that the client is participating only in this year. So, if a client is participating since September 2013, the field 'MonthsParticipatingThisYear' should only display '11' and not '15' (adding up all months since start). I don't know if I am making myself clear on this point?
Thank you so much,