Atomic Clock Time
Displayed in a variety of useful formats. Site is a front-end to U.S. government atomic clocks.
http://www.atomictime.net

Calendar Zone
A categorized collection of calendar related sites.
http://www.calendarzone.com

English Calendar
Includes an ecclesiastical calendar and a section on old and new style dating.
http://www.albion.edu/english/calendar

NIST Time and Frequency Division
The National Institute of Standards and Technology maintains time and frequency standards for the United States.
http://www.boulder.nist.gov/timefreq

This Day In History
Today in history every day - often with pictures and sound.
http://www.HistoryChannel.com/today

Time FAQ
Frequently Asked Questions about time (from sci.astro news group)
http://www.faqs.org/faqs/astronomy/faq/part3

Time References
To set your watch or clock accurately using a time reference. Explains the various options for residents of North America.
http://htexplained.com/watches/TimeRef