The DateTime represented by the string '30/02/2013' is not supported in calendar 'System.Globalization.GregorianCalendar'

I'm trying to use Ethiopian Calendar and the date 30/02/2013 is a valid date since in Ethiopian Calendar every month has 30 days and one extra month with 5 days. I tried to search for EthiopianCalendar in System.Globalization but I'm not able to get it, so how can I solve this issue?