Well, this is rather curious.

I wondered whether you might have misspelled hodiernal--you hadn't--by first checking out the spelling of diurnal and then looking for hodiurnal. No hodiurnal. But There was hodiernal.

So I wonder. I would expect diurnal to have been the parent term, but maybe not? Why the spelling change between diur and dier? I can vaguely recall one of Britten's songs from "A Ceremony of Carols" and the word hodie used in one of the carols. Any connection?