This word is derived from the Latin term “ultimo mense,” which directly translates to “in the last month.” The modern-day calendar used throughout most of the world is called the Gregorian calendar, and it took effect as a replacement for the Julian calendar in October 1582. The Gregorian calendar is solar, meaning it’s based on the progression of the seasons and the Earth’s orbit around the sun. Both the Julian and Gregorian calendars are divided into 12 months. The word “ultimo,” meaning “of last month,” can be compared with “instant,” meaning “in the current month,” and “proximo,” meaning “of next month.”