What are the hyphenation rules for hour-a-day? I realize that most English speakers in the US don't care, but I have been corrected on it by teachers. I suppose running into it in differential with related rates has ingrained it into me that it should be done in formal writing...
The way it was explained to me was that you needed the hyphenation since taking the words individually doesn't make sense. It would be an abbreviated form of a "hour every day."