date(1)には書式文字列はstrftime(3)に載っているっ
て書いてあるけど。strftime(3)は遠回りでもなんでも
なく、直球だと思う。まぁ、date(1)にstrftime(3)を
見ろと載っていると一言あってもいいけど。

An operand with a leading plus (`+') sign signals a user-defined format
string which specifies the format in which to display the date and time.
The format string may contain any of the conversion specifications
described in the strftime(3) manual page, as well as any arbitrary text.

つーか、質問者以外が難癖つけすぎ。