The Hebrew, or Jewish, calendar is both a solar and lunar calendar, as opposed to the Gregorian, or civil, calendar which is based on a solar year that is divided into 12 months. The Gregorian year 2015 corresponds to year 5775 of the Hebrew calendar.
In the Hebrew calendar, months follow the lunar cycle. A lunar month has about 29.5 days. Because the month is not divided into full days, the lunar months in the Hebrew calendar have either 29 or 30 days. Hebrew days begin at nightfall.
12 lunar months add up to only 354.4 days, as opposed to a solar year, which is made up of 365.25 days. To make up the 11-day difference and maintain a solar year, the Hebrew calendar has periodic leap years, which add an extra 30-day month to the end of the year. This ensures that the months correspond to the seasons of the year. The leap year occurs about once every three years.
Hebrew years begin counting from the moment of creation as interpreted from the Torah. This number is determined by adding the ages of people in the Bible back to creation. To find a corresponding Hebrew date from a Gregorian year, add 3760 to the Gregorian date. Add 3761 if the date falls after Rosh Hashana.