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

A History of Time
Explains the importance of concept of time in the history of mathematics, science and measurement.
http://www-history.mcs.st-andrews.ac.uk/HistTopics/Time_1.html

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

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

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/

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

International Standard Date and Time Notation
An overview of the ISO 8601 notation for dates.
http://www.cl.cam.ac.uk/~mgk25/iso-time.html

Internet Encyclopedia of Philosophy: Time
Time has been studied for thousands of years. Nevertheless, many issues remain to be resolved. This article explores both what is now known about time and what is controversial and unresolved.
http://www.iep.utm.edu/t/time.htm

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/

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

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

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

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

U.S. Time Zones
History of standard time in the U.S.
http://aa.usno.navy.mil/faq/docs/us_tzones

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

Wikipedia: Time
Time is a common term for the experience of duration, and a fundamental quantity of measuring systems. Article explains time definitions, history of measurements, perceptions in religion, philosophy and science.
http://en.wikipedia.org/wiki/Time

Z-Time
Article on the time at 0 degrees longitude. Covers the timezone origin, GMT, UTC, GCT, ZULU and JIG.
http://www.maybeck.com/ztime/