Credits where they are due:
Concept and Feriæ: Lord RandomFactor Systems Essential Info 1.1 for DOS, originally released around August 1988. Also the tides (probably inaccurate) and the three lunar numbers on the top right; I still have no idea what those mean. I was pretty sure that I first found this as STARTUP.EXE from the Freeware Hall of Fame BBS in the early 90s, but it's not on the
list of files on freewarehof.org NOTE: Version 1.1 did not include Endless September, Chinese, or Discordian calendars, and I made minor changes to the Feriæ.
Moon Phase calculator: Google Code repository of public-domain Java version - source code contains more attribution within.
Info on Mayan Calendar (Long Count, Haab, Tzolk'in): Smithsonian National Museum of the American Indian
Info on French Republican Calendar: Wikipedia
Info on Discordian Calendar and the (revised) X-Day: Also Wikipedia
Chinese New Year dates: Wikipedia again. I only entered dates from 1970-2049. Outside of those years it's an estimate. Sorry.
Info on Julian Day: Wikipedia. I spend a lot of time there. Also, the calculation of the fractional day is kind of kludgey.