You've made a very good point that proficiency in developing compiler infrastructure does not imply that you're qualified to develop every specific aspect of a

You've made a very good point that proficiency in developing compiler infrastructure does not imply that you're qualified to develop every specific aspect of a standard library. Date/Time, as you pointed out, is a very good example of this. It's a very complex domain that requires specialised knowledge. I'll counter this by saying that one aspect of language design is choosing the scope of the project, and deciding how best to implement a standard library targeting the language's intended domain. If your language is designed to implement web servers, then developing a GUI library might be a poor investment. Consequently, if your language is designed for implementing system applications, then investing time and talent into developing things like FFI, Filesystem and GUI functionality are just the prerequisites to the language being useful in its intended domain.