This field expects a java.lang.String but the script returned a java.lang.Class

public class MyClass {

    public static main(String[] args) {

        def field = "Passed";

        def passTime = use(TimeCategory) {
            new Date() + 60.days
        }

        if (field.equals("Passed")) {
            String newPassTime = passTime.format('M/d/yy')
        }

    }
}

I would like the output to be a string but for some reason the script is returning a class.

How can I return a string from this code?