Leap year most years that are multiples of 4 are leap years. In each leap year, the month of February has 29 days instead of 28. Adding one extra day in the calendar every four years compensates for the fact that a period of 365 days is shorter than a tropical year by almost 6 hours. Some exceptions to this basic rule are required since the duration of a tropical year is slightly less than 365.25 days. The Gregorian reform modified the Julian calendar's scheme of leap years as follows: Over a period of four centuries, the accumulated error of adding a
Julian calendar ordinary years in the Roman Republican calendar, from January to December, were: Sacrobosco then thought that Julius Caesar added one day to every month except February, a total of 11 more days to regular months, giving the ordinary Julian year of 365 days. A single leap day could now be added to this extra short February: He then said Augustus changed this, by taking one day from February to add it to Sextilis, and then modifying the alternation of the following months, to: so that the length of "Augustus" (August) would not be shorter than (and therefore inferior to) the