DateTime.pm is quite usable. It's "under development" in the same sense that any reasonably large city's road system is "under development". Yeah, there might be some major road construction going on, but that doesn't stop it from being useful. In fact, it's various interfaces (with both the application developer, and with the various formatting and calendar modules) are quite well defined, so we can expect there to be only minor annoyances in comparison to road construction.