I don't know about consistency within the standard library, but NewTimer's behavior definitely sounds consistent with the concept of distinguishing programming errors from exceptional conditions. Passing a negative number as a duration could either be defined behavior (equivalent to zero for programmer convenience and consistency with the principle that timers can fire late) or a programming error (should've checked that whatever you were subtracting to get that duration isn't backwards), but it's not an exceptional condition that can't be predicted in advance. Indeed, checking for a negative number, which is rarely necessary, is no more code than the 'if err' thing, and if it isn't necessary in a particular use of the function, you have the luxury of knowing it can't fail, and you don't need to figure out what will happen if it does.

(I bet you'd be unhappy at Rust irrecoverably panicking the thread for simply performing addition, if the result overflows!)