Even this system has its flaws, however, as the Julian calendar requires an extra day every four years (leap day) because the earth actually takes a little longer than 365 days in its trip around the sun (an extra five hours, forty-eight minutes, and forty-nine seconds, to be exact).

Even the leap day does not completely compensate for these extra hours, so there was a change to the 'Gregorian' calendar in 1582. In addition to repairing the gap and changing the start of the year to January 1, the new calendar also decreed that leap years would not occur on years divisible by 100 unless the year was also divisible by 400 (i.e., 1900 was not leap year, despite being divisible by four, but the year 2000 is a leap year after all).

All dates in this unit (and this semester) will be from the western calendar. The basic form of this calendar came from the Romans and is called the Christian or Gregorian calendar. It is based on the life of Christ, either the years before Christ, BC, of the years after Christ, AD. AD stands for Anno Domini, or 'year of our lord'. Many times, you will see the term CE instead of AD. CE stands for Common Era and has no connection to religion as AD does. CE is the same as AD. BCE is the same as BC.