This does not mean that years were counted the way we do now. They were counted from the start of the reign of the Emperor or Caesar and reset to one when the next Emperor took over. For more information on how the years are counted see the page counting years. Historians sometimes counted years ab urbe condita, that is since the founding of Rome.
The old Roman calendar was very complicated and required a group of men, known as the pontiffs, to decide when days should be added or removed to keep the calendar in track with the seasons. This made planning ahead difficult and the pontiffs were open to bribery in order to prolong the term of elected officials or hasten elections. In order to avoid these problems Julius Caesar abolished the use of the lunar year and the intercalary month, and regulated the civil year entirely by the sun. With the advice and assistance of Sosigenes, he fixed the mean length of the year at 365 1/4 days, and decreed that every fourth year should have 366 days, the other years having each 365. In order to restore the vernal equinox to the 25th of March, the place it occupied in the time of Numa, he ordered two extraordinary months to be inserted between November and December in the current year, the first to consist of thirty three, and the second of thirty-four days. The intercalary month of twenty-three days fell into the year of course, so that the ancient year of 355 days received an augmentation of ninety days; and the year on that occasion contained in all 445 days. This was called the last year of confusion. The first Julian year commenced with the 1st of January of the 46th before the birth of Christ, and the 708th from the foundation of the city.
In the distribution of the days through the several months, Caesar adopted a simpler and more commodious arrangement than that which has since prevailed. He had ordered that the first, third, fifth, seventh, ninth, and eleventh months, that is January, March, May, July, September and November, should each have thirty-one days, and the other months thirty, except February, which in common years should have only twenty-nine day, but every fourth year thirty days. This order was interrupted in 8 BC to gratify the vanity of Augustus, by giving the month bearing his name as many days as July, which had been re-named after the first Caesar during 44BC. A day was accordingly taken from February and given to August; and in order that three months of thirty-one days might not come together, September and November were reduced to thirty days, and thirty-one given to October and December.
The additional day which occurred every fourth year was given to February, being the shortest month, and was inserted in the calendar between the 24th and 25th day. February having then twenty-nine days, the 25th was the 6th of the calends of March, sexto calendas; the preceding, which was the additional or intercalary day, was called bis-sexto calendas,--hence the term bissextile, which is still employed to distinguish the year of 366 days. The English denomination of leap year would have been more appropriate if that year had differed from common years in defect, and contained only 364 days. In the modern calendar the intercalary day is still added to February, not, however, between the 24th and 25th, but as the 29th.
In the Julian calendar, the tropical year is approximated as 365 1/4 days = 365.25 days. This gives an error of 1 day in approximately 128 years.
The approximation 365 1/4 is achieved by having 1 leap year every 4 years (as explained above) and the rule for calculation is that every year divisible by 4 is a leap year.
However, this rule was not followed in the first years after the introduction of the Julian calendar in 45 BC. Due to a counting error, every 3rd year was a leap year in the first years of this calendar's existence. The leap years were:
45 BC, 42 BC, 39 BC, 36 BC, 33 BC, 30 BC, 27 BC, 24 BC, 21 BC, 18 BC, 15 BC, 12 BC, 9 BC, AD 8, AD 12, and every 4th year from then on.There were no leap years between 9 BC and AD 8. This period without leap years was decreed by emperor Augustus and earned him a place in the calendar, as the 8th month was named after him.
It is a curious fact that although the method of reckoning years after the (official) birth year of Christ was not introduced until the 6th century, by some stroke of luck the Julian leap years coincide with years of our Lord that are divisible by 4.
The seven day week was introduced by the Emperor Constantine I in the 4th century AD.
In order to remedy this, two steps were necessary: 1) The Julian calendar had to be replaced by something more adequate. 2) The extra days that the Julian calendar had inserted had to be dropped.
The solution to problem 1) was the Gregorian Calendar.
The solution to problem 2) depended on the fact that it was felt that 21 March was the proper day for vernal equinox (because 21 March was the date for vernal equinox during the Council of Nicaea in AD 325). The Gregorian calendar was therefore calibrated to make that day vernal equinox. By 1582 vernal equinox had moved (1582-325)/128 days = approximately 10 days backwards. So 10 days had to be dropped.
Joseph Justus Scaliger (1540--1609) was a noted Italian-French philologist and historian who was interested in chronology and reconciling the dates in historical documents. As many calendars were in use around the world this created the problem of which one to use. To solve this Scaliger invented his own era and reckoned dates by counting days. He started with 4713 BC January 1 because that was when solar cycle of 28 years (when the days of the week and the days of the month in the Julian calendar coincide again), the Metonic cycle of 19 years (because 19 solar years are roughly equal to 235 lunar months) and the Roman indiction of 15 years (decreed by the Emperor Constantine) all coincide. There was no recorded history as old as 4713 BC known in Scaliger's day, so it had the advantage of avoiding negative dates. Joseph Justus's father was Julius Caesar Scaliger, which might be why he called it the Julian Cycle. Astronomers adopted the Julian cycle to avoid having to remember "30 days hath September ...." and to avoid the 10/11 day hiatus in the Gregorian calendar.
For reference, Julian day 2450000 began at noon on 1995 October 9. Because Julian dates are so large, astronomers often make use of a "modified Julian date"; MJD = JD - 2400000.5. (Though, sometimes they're sloppy and subtract 2400000 instead.)
Thanks to Claus Tondering and William Hamblen for some of the above information.