The Duration class is missing features that the std::chrono::duration class provides, which makes it cumbersome to work with and difficult to seek outside help for (bc there is a mongo-specific Duration lib rather that the standard library). A recent example is linked in the comments, where I had to go through the 
std::chrono::duration API in order to achieve my goal of casting a double to a duration type.
 
It would be useful to do a refresh of the Duration class to make it's API more aligned with std::chrono::duration in order to improve developer velocity & intuition about the class.