I can see how this happens, months have been standardised.
I set the Start date to 24/09/2014 and the Due date to 23/09/2016 on a topic.
Duration: 523 workdays, 104 weeks or 26 months!
Entering a Duration of 24 months gives a due date of 26/07/2014.
Perhaps a more complex calculation should be performed to match the calendar month. What do you think?
I was using this to track a bi-annual domain name renewal.