This manifests itself In the Node REPL, where #inst "1582-10-01" parses into an inst that has the date as 11:
In Clojure, when the default java.util.Date is employed, #inst value strings are interpreted using Java's GregorianCalendar, essentially a hybrid Julian / Gregorian system.
This effectively means that for dates prior to the Julian to Gregorian transition, the string representations are interpreted differently.
The above assertion is incorrect. Also, #inst "1500" yields a date in 1499. Assigning back to me...
It does this in a way that allows macro authors to opt into the same machinery for #inst values embedded in macros, if desired: They could use the cljs.instant facility and bind *data-readers*.
#inst in edn/clojure is about a representation for Gregorian calendar instants and does not prescribe a behavior for pre-Gregorian calendar dates. That is, it is considered undefined how pre-Gregorian insts are interpreted by a particular language.
Certainly, I think that all users would expect #inst to read/print roundtrip in a particular host.