ELI5 Calendars - how the world agrees on the day & 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

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