To simply convert between the Julian and Hebrew calendars, use an online calendar converter, such as the ones provides by Fourmilab.ch, CalendarHome.com and Midrash.org. On Fourmilab.ch and Midrash.org, find the Hebrew calendar section, enter the desired date and click Convert. On CalendarHome.com simply enter the desired Hebrew calendar date.
Fourmilab.ch, CalendarHome.com and Midrash.org also provide the ability to convert between many other dating systems, such as the standard Gregorian calendar, the Islamic calendar and even the Mayan calendar. All three websites also provide conversions for technical calendars, such as ISO-8601, Unix and Excel Serial Day Number.
The Hebrew calendar is a combination lunar-solar calendar. Months follow the lunar cycle of 29.5 days. However, as 12 lunar months equals 354 days, the Hebew calendar occasionally adds an entire month to keep the calendar in sync with the solar cycle of 365 days.
Called embolismic years, Hebrew calendar years with extra months occur within a 19-year cycle on the third, sixth and eighth years, as well as years 11, 14, 17 and 19. Both common years and embolismic years come in three versions: deficient, regular and complete. Deficient years lack a day, and complete years add a day.
Hebrew calendar days start at sunset, which is why Jewish holidays start the evening before the calendar day. Hebrew-calendar hours divide each day's daylight time into 12 equal parts, meaning the absolute length of hours varies throughout the year.