In the functions and operators specification the following two signatures are given

op:subtract-dateTimes($arg1 as xs:dateTime, $arg2 as xs:dateTime) as xs:dayTimeDuration?

and 

op:subtract-dates($arg1 as xs:date, $arg2 as xs:date) as xs:dayTimeDuration?

but I am puzzled as to why these specify a return type of "xs:dayTimeDuration?". Since neither argument can be the empty sequence, and since the static context timezone is assumed if there isn't one on one of the arguments, I don't see how these functions can fail. Additionally, the rest of the operator backers do not return empty sequences. Should the return type on these two functions be "xs:dayTimeDuration"? If not, there should be an example of why not in the function description.