I would like to be able to convert a Java date format string, e.g. dd/MM/yyyy (07/06/2009) to a JavaScript date format string, e.g. dd/mm/yy (07/06/2009).
If you just want to format dates my date extensions will do that well - it also parses data formats and does a lot of date math/compares as well:
DP_DateExtensions Library
Not sure if it'll help, but I've found it invaluable in several projects.