Who maintains what time it was in Yugoslavia in 1970? Or Serbia? What country maintains the time information for the island of Taiwan? Or Hong Kong while under British rule or while under Japanese rule?

It might be possible to use that for the information of now - to answer the question of "what is local time for me based on UTC?" or "what is local time for someone else now?" ... but what about the information of yesterday? When it was 12:01 PM in Chicago in 1948, what time was it in Hong Kong?