The source code is available. I've never formally learned Java, but it all looks reasonably simple and clear -- not a pile of spaghetti like some apps.

There appears to be support for activity "Duration" in the code, but it's just missing from the "Edit Appointment" screen (or whatever that's called). So I guess I'll put that aside as yet another project to work on if/when I ever stop being so darned busy with paid work.

Edit: There's also j2cal, which is a fork of ancal that adds Duration to the GUI along with other minor tweaks. Unfortunately they've broken the "Day" view -- teensy tiny font on my handset. That's probably easy to fix though, if I can find the source code.