The Calendar
Summarizes the history of various calendars developed and used over the centuries.
http://www.skeptics.com.au/journal/calendar.htm

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/

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

Time Zone and Daylight Saving Time Data
A directory of sites providing descriptions of civil timekeeping concepts, source code, databases, and maps.
http://www.twinsun.com/tz/tz-link.htm

A Walk Through Time
Evolution of timekeeping through the centuries.
http://physics.nist.gov/GenInt/Time/time.html

Today's Calendar and Clock Page
Information on all things relating to calendars, dates, holidays, and time. Find today's date on several different cultural and religious calendars. Includes section on Celestial data, countdown clocks, and information on the dates past events.
http://www.ecben.net/calendar.shtml

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

United States Code
Weights, measures and standard time.
http://tycho.usno.navy.mil/260.html

Time
Provides descriptions of several standards of time.
http://scienceworld.wolfram.com/astronomy/Time.html

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

Calendar Studies
Information about calendar history and calendar reform.
http://www.hermetic.ch/cal_stud.htm

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/

A Few Facts Concerning GMT, UT, and the RGO
Explains the differences between several time systems.
http://www.apparent-wind.com/gmt-explained.html

Infoplease: Daylight Saving Time
Information and history about Daylight Saving Time all over the world.
http://www.infoplease.com/spot/daylight1.html

NPR : 'Seize the Daylight': A History of Clock Chaos
Benjamin Franklin first called for the government to tinker with clock hours in the 1780s. But it wasn't until World War I that the United States adopted daylight-saving time as a way to get more efficiency out of the day. "Seize the Daylight" Author David Prerau talks about the complicated politics and curious history of DST, and he shares an excerpt from his book. [4:54 streaming audio broadcast]
http://www.npr.org/templates/story/story.php?storyId=4572036

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

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