I am sure there is a better way, but I did this using a series of sub-strings:
Substring(String( JobHead.DueDate ,"99-99-9999"),7 ,4 ) + "-" + Substring(String( JobHead.DueDate ,"99-99-9999"),1 ,2 ) + "-" + Substring(String( JobHead.DueDate ,"99-99-9999"),4 ,2 ))