ELI5 Calendars - how does the world agree on the date?

Why did the Russian team arrive at the 1908 olympics 12 days late? How does the whole world agree on what day and date it is? Why did the Pope step in to replace the Julian calendar? Why is 2000 a leap year but 2100 isn't? What was the most recent country to adopt the Gregorian calendar.   ... We explain like i'm five!   Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: pawataka, momsemann, ibn_cthulhu, cptsaysin, innovator97 & dercatzfragger To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Send us an e-mail: [email protected]