Calendar websites:
Calendar at Wikipedia
Calendar Converter at Harvard
Books about calendars:
Calendrical
Calculations: The Millennium Edition
by Edward M. Reingold and Nachum Dershowitz
If you want to write software for calendars, this book is for
you! It gives algorithms for the following calendars: Gregorian
(current civil), ISO (International Organization for
Standardization), Egyptian (and nearly identical Armenian), Julian
(old civil), Coptic, Ethiopic, Islamic (Moslem), modern Persian
(both astronomical and arithmetic forms), Baha'i (both present and
future forms), Hebrew (Jewish), Mayan (long count, haab, and
tzolkin), Balinese Pawukon, French Revolutionary (both astronomical
and arithmetic forms), Chinese (and nearly identical Japanese), old
Hindu (solar and lunisolar), and modern Hindu (solar and
lunisolar).
Calendrical
Tabulations 1900-2200
by Edward M. Reingold and Nachum Dershowitz
This book has tables showing the dates for the following
calendars: Gregorian, ISO, Hebrew, Chinese, Coptic, Ethiopic,
Persian, Hindu lunar, Hindu solar, and Islamic.
Marking
Time: The Epic Quest to Invent the Perfect Calendar
by Duncan Steel
The author gives a history of western calendars that is amazingly
rich in detail and sprinkled with wit and humor. An excerpt from
the jacket:
Did you know?...
- Russia's October Revolution in 1917 actually occurred on November
7th
- For centuries, Britain and the colonies rang in the New Year on
March 25th
- The Roman Empire originally observed an eight-day week
- The Roman anno Domini (A.D.) year-counting system is
wrong, and Jesus' birth actually occurred some years before
December 25th, 1 B.C.
These are just a few of the little-known facts that you will
find in acclaimed author Duncan Steel's eye-opening chronicle of
the calendar.
Jewish Calendar Mystery Dispelled
by George Zinberg
This book is out of print, but try this link:
Google search
This is the book I used to write the Hebrew calendar code used in
Rosetta Calendar.