RFC 3339 should be preferred to iso, but it does not solve the wall clock issue. In a calendar application I worked on, we use city name and lat/lng + "naive" timestamp (yyyy-mm-dd hh:mm or moon based date or other specialties like Japanese year depending on the locale). And we "resolve" to UTC + offset when the date is close.

https://news.ycombinator.com/item?id=37339535

https://www.mooncratertycho.com/the-12-lunar-calendars-still...