I have a couple of calendar programs on my list — not scheduling or appointment tools, but calendars — and I want to be able to show slight differences between each.
I can’t remember meeting a distribution that didn’t include cal, and I probably never will. In Arch it’s part of util-linux, but I’m not sure about Debian (it doesn’t show up in the list of files for util-linux for squeeze).
cal is probably a good example of a Unix-ish program — it’s small, it only does one thing, it is easily filtered and redirected, etc.
On the other hand, what you see above is about 98 percent of what cal does. Not inadequate, just the bare minimum.
So I suppose if you need to know when the next month with a Friday the 13th, or what day of the week your birthday was, cal will do the job with gusto.
But after that … ?
And by the way, this is not an error.