JULIAN DAYS................ [Se preferir, veja a página em Português com mais detalhes]
Insert a date – from 4713 Jan 1st to 1 Dec 31st B.C. or from 1 Jan 1st to 1582 Oct 4th or from 1582 Oct 15th to 9999... Dec 31st A.D. – and see the correspondent "julian day" (examples: |4713|1|1|bc| or |2002|12|9|ad|) - notice that 1582 Oct 5 to 14 A.D. were suppressed in the transition to Gregorian Calendar:
Insert a second date and see the elapsed days between both (don't insert the second date if you don't have the first one):
Contrary to that, insert a "julian day", from 0 to infinity, and see the correspondent date:
Note: If you compare this calendar with the one in Microsoft/Office/Excel, you will find a difference in year 1900. This is a problem in Excel, that incorrectly considers 1900 a leap year. Any attempt to calculate former dates (before 1900 March 1st), starting from Excel calendar, will result 1 day mistake. The bug comes from Lotus 1-2-3 and was purposely implemented in Excel, in order to match both spreadsheets.
See details on my correspondent page – portuguese version.