No, the 12 month system was previously used by the Julian calendar.
Our current calendar comes for the Julian calendar, the calendar introduced by Julius Caesar. In the 15th century pope Gregory XIII shortened the day of that calendar by about 11 minutes. Apart from that, our calendar is the same as the one introduced by Julius Caesar. Because of this, the name of our current calendar is Gregorian calendar. The Roman calendar was divided into months and the name of the months we use today are derived from the names the Romans used. For a short while at the beginning of their history, the Romans had calendar with 10 months. Soon after that, it was reformed and lengthened to 12 months. The Julian Calendar was a further reform of the Roman calendar. Two months were renamed after Julius Caesar and Augustus. This is the origin of the names of the months of July and August. The names of the other months came from the older Roman calendar.
All of the English month names are based on the Latin names of the Roman months. The calendar we use is the Gregorian calendar. It derives its name from Pope Gregory XIII who introduced some minor modifications to the Julian calendar in the 16th century. This means that we use a slightly modified version of the calendar introduced by Julius Caesar.
Pope Gregory XIII revised the Julian calendar in 1582.
It was the old Roman calendar which had only 10 months in a year.
he Roman calendar has the same months and month lengths as the Julian calendar, but inserts leap days according to a different rule
The Roman Calendar has 10 months in its system. Among these ten months, there were 304 days. This has been expanded on to 12 months and 365 days, which is still used today.
The First Roman calendar was the Romulean calendar, established by Romulus, the first king of Rome, when he founded Rome in 753 BC. It only had 10 months. The second calendar, the calendar of Numa, was established soon afterwards by the second king of Rome, Numa Pompilius. It had 12 months. Both these calendars were lunar. The Julian calendar was established by the Julius Caesar in 45 BC. He switched from a lunar to a solar calendar. Apart from some minor modifications introduced by Pope Gregory XIII in the 15th century AD, this is the calendar we still use today.
January and February
Pope Gregory mandated the use of the Gregorian calendar on February 24, 1582.
Two examples of Roman names in the calendar are for the months of Mars and June. Mars was the Roman god of war, and Juno was the goddess of marriage.
The classical Roman calendar was originally lunar, but later developed into a similar system to the modern one (in fact it was a precursor to the current Julian calendar) using months of either 30 or 31 days.
Apart from some minor changes introduced by Pope Gregory XIII in 1478,the calendar we still use today is the calendar which was created by Julius Caesar. Caesar reformed the Roman calendar, switching from a lunar to a solar one.