Takes a java.util.Date or joda DateTime or js/Date
instance and returns milliseconds since UNIX epoch. Returns a nil if passed none of those.

Converts clojure datastructure to json

assocs a val of nil to the given key if the map doesn't contain key

Similar to how leiningen merges profiles:
keys are assoc'd, vectors are appended, sets are unioned.
:replace metadata on b causes the contents of b to completely override a.