The Roman era began with the foundation of Rome in 753 b.c. By 46 b.c., various imperfections in the Roman calendar had caused it to fall into confusion and Julius Caesar then reformed it.
The year 46 b.c. was made to consist of 445 days, and is called "The Year of Confusion". Thereafter, each year consisted of 365 days except that every fourth year was a leap year. This Julian, or Old Style calendar remained in general use in Europe until 1582.
By 1582, there was a difference of ten days between the Julian calendar and the tropical year. In that year, Pope Gregory XIII ordered that October 5th should be called October 15th, and that during end of the century (00) years, only the fourth should be a leap year. This Gregorian or New Style calendar is still in use today.
It was adopted by Britian (including North America and the colonies) in 1752 (September 3rd being called September 14th) and the beginning of the official year altered from March 25th (date of the Vernal Equinox when Julian introduced his calendar) to January 1st.