AM

   1. abbreviation for the Latin ante meridiem, "before noon," used after a time to indicate that the time is before 12:00 (noon). The notations "am" and "pm" are used extensively in the United States, Canada, and other countries where time is usually not stated on a 24-hour basis. By convention, midnight is represented as 12:00 am and noon as 12:00 pm. In text, however, it is best to avoid the use of 12:00 am or 12:00 pm since the reader may not be aware of these conventions.

   2. abbreviation for the Latin anno mundi, "year of the world." This abbreviation is traditionally used to designate years in the Jewish calendar, which counts years from the creation of the world as described in the Hebrew scriptures. The Jewish calendar is lunisolar; its years correspond to ordinary solar years (see month²). The first day of AM 1 is equivalent to 6 October 3761 BCE in the Julian calendar, and AM 5769 began at sunset of 29 September 2008 in the current (Gregorian) calendar.

   Link: Wikipedia has a good description of the Jewish calendar and its history; Remy Landau has an extensive web site on the subject; and a date converter is available.