The Julian calendar is used for all dates up to 1582 Oct 04. After that date, the Gregorian calendar is used. Due to the Gregorian Calendar reform, the day after 1582 Oct 04 (Julian calendar) is 1582 Oct 15 (Gregorian calendar). Note that Great Britain did not adopt the Gregorian calendar until 1752. For more information, see Calendars.
The Julian calendar does not include the year 0, so the year 1 BCE is followed by the year 1 CE. This is awkward for arithmetic calculations. All pages in this web site employ the astronomical numbering system for dates (they use the year 0). Years prior to the year 0 are represented by a negative sign. Historians should note that there is a difference of one year between astronomical dates and BCE dates. Thus, the astronomical year 0 corresponds to 1 BCE, and year -100 corresponds to 101 BCE, etc.. (See: Year Dating Conventions )
There is some historical uncertainty as to which years from 43 BCE to 8 CE were counted as leap years. For the purposes of this web site, we assume that all Julian years divisible by 4 are be counted as leap years.