There has been a proposal in the 1950s to the UN by a German mathematician to replace the current calendar by a decimal-based system, in which leap YEARS are not needed either: everything would be divisible by 100. I can't remember where I heard this from, but the anecdote goes he got a reply saying thanks for the proposal, but it is not feasible to introduce such a massive change globally, irrespective of the proposed improvements.
That is mixing up quite a few different things. World Calendar [1] was proposed in League of Nations/United Nations, but it was created by US person. There was slightly earlier proposal, International Fixed Calendar[2] from British person that also had some popularity. Neither of these were decimal calendars though, that is something from French revolution [3], and even they did not really manage to make it very decimal.
Afaik the most vocal opposition to reforms came from the US
