Conversation so far:
are there any countries that don't use the gregorian calendar
Countries which do not use the Gregorian calendar include Afghanistan and Iran (which use the Solar Hijri calendar), Ethiopia (Ethiopian calendar), and Nepal (Vikram Samvat).
who invented the gregorian calendar
The Gregorian calendar is named after Pope Gregory XIII, who introduced it.

Question: when was the system invented
Self-contained rewrite: when was the gregorian calendar invented