Convert Gregorian dates to Julian or vice versa:
The Julian calendar was used ubiquitously until only about 350 years ago. Different countries in Europe adopted the Gregorian calendar at different times. The last one to do so as far as I know was Russia in 1917. This page can help you convert dates between Julian and Gregorian calendars. You can then go back and compute the Hebrew equivalent if you only know the Julian date.
It is interesting that Julian and Gregorian calendars correspond respectively to the solar calendars proposed by Rabbi Ada and Shmuel in the Talmud. This solar calendar controversy continued from then on for two thousand years in every country where solar calendars were used. Today Rabbi Ada's calendar (on whose principles Julian calendar is based) is still used in Jewish law in certain cases. Most notably, we start praying for rain on December 4th, which was the 60th day after the autumn equinox in that calendar, even though it is now off by about 2 weeks. Thus the Julian calendar is not entirely obsolete from the Jewish perspective.
Sorry, we did not provide a direct conversion function between Hebrew and Julian calendars; most users never really need this. Please let us know if you find any problems.