The Ethiopian calendar is similar to the Hebrew (solar) calendar in that it is made up of twelve thirty day months but also with a thirteenth month of either five or six days, depending on leap years. The start of the Ethiopian calendar was August 29, 7 (Julian). But while the Hebrew calendar adds extra, "uncounted" days at the end of the sixth month to cause the first day of the seventh month to fall on a weekly sabbath with the remaining days after the twelvth month to fill out the year until the next vernal equinox, the Ethiopian calendar puts all the extra days at the end of their year as the thirteenth month. The Coptic calendar is identical in structure to the Ethiopian, but has different year numbers and month names. Also the Ethiopian clock is six hours back (see http://www.abyssiniacybergateway.net/fidel/l10n/ under "Formatting Dates and Times") with our six a.m. being their '0'th hour.

http://rs6.loc.gov/frd/cs/ethiopia/et_glos.html - Library of Congress - Glossary on Ethiopia

Ethiopian calendar year
   The Ethiopian year consists of 365 days, divided into twelve months of thirty days each plus one additional month of five days (six in leap years). Ethiopian New Year's falls on September 11 and ends the following September 10, according to the Gregorian (Western) calendar. From September 11 to December 31, the Ethiopian year runs seven years behind the Gregorian year; thereafter, the difference is eight years. Hence, the Ethiopian year 1983 began on September 11, 1990, according to the Gregorian calendar, and ended on September 10, 1991. This discrepancy results from differences between the Ethiopian Orthodox Church and the Roman Catholic Church as to the date of the creation of the world.

http://www.cae.wisc.edu/manuals/gnu/emacs/emacs.Calendar_Systems.html -

http://www.ethiopic.com/eth_calt.htm - Ethiopian calendar software with web simulation

http://erebus.phys.cwru.edu/~copi/calendar.html - ...