ScriptManager and DateTime

I am using the ScriptManager to declare a ServiceReference to a specific ASMX file. Later on in the JavaScript I call one of the methods to save some data. Everything works fine when I pass it a US formatted date. However, when I pass an Australian formatted date it does not work anymore.
What do I need to do to make the service aware that sometimes I want to pass an AU date and other times pass a US date? Is there some sort of Culture property I need to set?

Thanks

...