Sigh. I don't know how long I looked for a simple solution before I gave up and wrote one, but somehow I never saw the DateTime option. Still, I think it's worth publishing the algorithm, even if it's only a benefit for people who need to implement it in something other than perl...

I might simplify some code next week.

PS - I wonder why I can see the module on CPAN, but not via PPM in any of Active's repositories