It adds an "iso" method to Class::Date which returns an ISO-style string.
But that ability is already in Class::Date. On just needs to set the desired format and use the string method.
I'm rather iffy on the means it uses to add the method to Clas::Date.Rather than inherit the class it specifies a new Class::Date package with the added method. (Admittedly I've never seen this style of adding a method to a class, but it seems dodgy to me.)