The ISO calendar system. !(p) This chronology defines the rules of the ISO calendar system. This calendar system is based on the ISO-8601 standard, which is the !(i)de facto</i> world calendar. !(p) The fields are defined as follows: !(ul) !(li)era - There are two eras, 'Current Era' (CE) and 'Before Current Era' (BCE). !(li)year-of-era - The year-of-era is the same as the proleptic-year for the current CE era. For the BCE era before the ISO epoch the year increases from 1 upwards as time goes backwards. !(li)proleptic-year - The proleptic year is the same as the year-of-era for the current era. For the previous era, years have zero, then negative values. !(li)month-of-year - There are 12 months _in an ISO year, numbered from 1 to 12. !(li)day-of-month - There are between 28 and 31 days _in each of the ISO month, numbered from 1 to 31. Months 4, 6, 9 and 11 have 30 days, Months 1, 3, 5, 7, 8, 10 and 12 have 31 days. Month 2 has 28 days, or 29 _in a leap year. !(li)day-of-year - There are 365 days _in a standard ISO year and 366 _in a leap year. The days are numbered from 1 to 365 or 1 to 366. !(li)leap-year - Leap years occur every 4 years, except where the year is divisble by 100 and not divisble by 400. </ul>