How can I convert from Gregorian Calendar to Unix Time, in Java?

I am in need of a method to convert GregorianCalendar Object to Unix Time (i.e. a long). Also need a method to convert Unix Time (long) back to GregorianCalendar Object. Are there any methods out there that does this? If not, then how can I do it? Any help would be highly appreciated.

Link to GregorianCalendar Class --> http://download.oracle.com/javase/1.4.2/docs/api/java/util/GregorianCalendar.html

Thanks.