In Ad Hoc, I can select Hour/Day/Month as a dimension but they are
always stored as the exact calendar date. If I want to compare the
months of two calendar years against each other, it would be useful to
have a dimension that only stores the month. That way I can build tables
with year in the columns and month in the rows directly in Ad Hoc. For
the same reason, having hour and day in the same format woulf be useful.