Conversation so far:
where do the prime meridian and the international date line meet
The prime meridian separates the eastern hemisphere from the western hemisphere. Halfway around the world, at 180 degrees longitude, is the International Date Line
what is the offical prime meridian
There is an international agreement that the meridian that runs through Greenwich, England, is considered the official prime meridian.

Question: when was the official line decided
Self-contained rewrite: when was the official prime meridian chosen