It's probably a date that a country adopted something or reached a certain milestone.
Note how four out of five of the so-called "communist states", as well as Myanmar, are in grey. Presumably being a socialist/Juche One-Party State/Junta has something to do with these countries not having this thing. The notable exception is Vietnam.
Note how four out of five of the so-called "communist states", as well as Myanmar, are in grey. Presumably being a socialist/Juche One-Party State/Junta has something to do with these countries not having this thing. The notable exception is Vietnam.