Although Greek astronomers had known, at least since Hipparchus, a century before the Julian reform, that the tropical year was slightly shorter than 365.25 days, the calendar did not compensate for this difference. As a result, the calendar year gains about three days every four centuries compared to observed equinox times and the seasons. This discrepancy was largely corrected by the Gregorian reform of 1582. The Gregorian calendar has the same months and month lengths as the Julian calendar, but, in the Gregorian calendar, year numbers evenly divisible by 100 are not leap years, except that those evenly divisible by 400 remain leap years. (Even then, the Gregorian calendar diverges from astronomical observations by one day in 3,030 years). The difference in the average length of the year between Julian (365.25 days) and Gregorian (365.2425 days) is 0.002%, making the Julian 10.8 minutes longer. The accumulated effect of this difference over some 1600 years since the basis for calculation of the date of Easter was determined at the First Council of Nicea means for example that, from 29 February "Julian" (13 March "Gregorian") 1900 and until 28 February "Julian" (13 March "Gregorian") 2100, the "Julian" calendar is 13 days behind the "Gregorian" calendar; one day after (i.e. on 29 February "Julian" or 14 March "Gregorian"), the difference will be 14 days. Although the new calendar was much simpler than the pre-Julian calendar, the pontifices initially added a leap day every three years, instead of every four. There are accounts of this in Solinus, Pliny, Ammianus, Suetonius, and Censorinus. Macrobius gives the following account of the introduction of the Julian calendar:  So, according to Macrobius,  Some people have had different ideas as to how the leap years went. The above scheme is that of Scaliger (1583) in the table below. He established that the Augustan reform was instituted in 8 BC. The table shows for each reconstruction the implied proleptic Julian date for the first day of Caesar's reformed calendar (Kal. Ian. AUC 709) and the first Julian date on which the Roman calendar date matches the Julian calendar after the completion of Augustus' reform. Alexander Jones says that the correct Julian calendar was in use in Egypt in 24 BC, implying that the first day of the reform in both Egypt and Rome, , was the Julian date 1 January if 45 BC was a leap year and 2 January if it was not.