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.
when was the gregorian calendar invented
The Gregorian calendar was introduced in October 1582.
why was the gregorian calendar invented
The Gregorian calendar was developed as a correction to the Julian calendar, shortening the average year to stop the drift of the calendar with respect to the equinoxes.

Question: Are there extra days
Self-contained rewrite: does the gregorian calendar have leap years