<P> The Roman calendar was reformed by Julius Caesar in 45 BCE . The Julian calendar was no longer dependent on the observation of the new moon but simply followed an algorithm of introducing a leap day every four years . This created a dissociation of the calendar month from the lunation . </P> <P> In the 11th century in Persia, a calendar reform led by Khayyam was announced in 1079, where the length of the year was measured as 365.24219858156 days . Given that the length of the year is changing in the sixth decimal place over a person's lifetime, this is outstandingly accurate . For comparison the length of the year at the end of the 19th century was 365.242196 days, while today it is 365.242190 days . </P> <P> The Gregorian calendar was introduced as a refinement of the Julian calendar in 1582, and is today in worldwide use as the de facto calendar for secular purposes . </P> <P> The term calendar itself is taken from the calends, the term for the first day of the month in the Roman calendar, related to the verb calare "to call out", referring to the calling or the announcement that the new moon was just seen . Latin calendarium meant "account book, register", as accounts were settled and debts were collected on the calends of each month . </P>

Who came up with the modern day calendar