Looks like NativeDate.safeToString() is not so safe:

jjs> new Date(NaN)
Exception in thread "main" ECMAScript Exception: RangeError: Invalid Date
at jdk.nashorn.internal.runtime.ECMAErrors.error(ECMAErrors.java:66)
at jdk.nashorn.internal.runtime.ECMAErrors.rangeError(ECMAErrors.java:275)
at jdk.nashorn.internal.runtime.ECMAErrors.rangeError(ECMAErrors.java:247)
at jdk.nashorn.internal.runtime.ECMAErrors.rangeError(ECMAErrors.java:234)
at jdk.nashorn.internal.objects.NativeDate.toISOStringImpl(NativeDate.java:1030)
at jdk.nashorn.internal.objects.NativeDate.safeToString(NativeDate.java:185)
at jdk.nashorn.internal.runtime.JSType.toStringImpl(JSType.java:870)
at jdk.nashorn.internal.runtime.ScriptRuntime.safeToString(ScriptRuntime.java:206)
at jdk.nashorn.tools.Shell.readEvalPrint(Shell.java:394)
at jdk.nashorn.tools.Shell.run(Shell.java:164)
at jdk.nashorn.tools.Shell.main(Shell.java:139)
at jdk.nashorn.tools.Shell.main(Shell.java:118)