The aggregation operators page says "Arithmetic operators support only numbers." But when using $subtract on two ISODates, it seemingly returns the number of milliseconds between them, which is decent, usable behavior. The page for $subtract doesn't note this either. http://docs.mongodb.org/manual/reference/operator/aggregation/subtract/#exp._S_subtract
This behavior should be documented on the pages of the arithmetic operators, unless there's a reason we don't want to consider this a feature?