All these functions in the reference manual have a sentence like:
(this is from DAY)
If the argument is a time duration or timestamp duration: The result is the day part of the value, which is an integer between -99 and 99. A nonzero result has the same sign as the argument.
This can be removed since Derby does not support durations. Then the surrounding text probably needs re-work as it leaves only a single type of argument.