It turns out that JavaScript's Date uses a proleptic Gregorian calendar, while Clojure's use of Date via #inst involves a hybrid Julian / Gregorian calendar. A hosty patch to fully embrace the JavaScript proleptic Gregorian calendar in ClojureScript is in https://clojure.atlassian.net/browse/CLJS-3291

I can’t really start to articulate how happy I am that you took over this bug.

@mfikes would be nice to see this as a part of EDN spec, then other implementations can do it too.