A problem with a format parameter is that duration cannot be broken down to months/years without specifying the start date
You don't have to include months/years in the format parameter. I just used that as an example with how mine worked when I included a format parameter.
@argv0, yes, $asctime($duration(),format) would also work. Even so, for such a commonly used identifier, I don't see why a format parameter can't be included in the identifier.
