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:

See details on my correspondent page – portuguese version.