These resources are a combination of actual time standards to dissertations on the history of timekeeping.
In Europe during most of the Middle Ages (roughly 500 CE to 1500 CE), technological advancement virtually ceased. Sundial styles evolved, but didn't move far from ancient Egyptian principles.
During these times, simple sundials placed above doorways were used to identify midday and four "tides" (important times or periods) of the sunlit day. By the 10th century, several types of pocket sundials were used. One English model even compensated for seasonal changes of the Sun's altitude.
Then, in the first half of the 14th century, large mechanical clocks began to appear in the towers of several large Italian cities. We have no evidence or record of the working models preceding these public clocks, which were weight-driven and regulated by a verge-and-foliot escapement. Variations of the verge-and-foliot mechanism reigned for more than 300 years, but all had the same basic problem: the period of oscillation of the escapement depended heavily on the amount of driving force and the amount of friction in the drive. Like water flow, the rate was difficult to regulate.
Another advance was the invention of spring-powered clocks between 1500 and 1510 by Peter Henlein of Nuremberg. Replacing the heavy drive weights permitted smaller (and portable) clocks and watches. Although they ran slower as the mainspring unwound, they were popular among wealthy individuals due to their small size and the fact that they could be put on a shelf or table instead of hanging on the wall or being housed in tall cases. These advances in design were precursors to truly accurate timekeeping. - NIST
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A Walk Through Time | physics.nist.gov/GenInt/Time/time.html
Ancient calendars, early clocks, world time scales and time zones, NIST time services and more, from the National Institute of Standards and Technology (NIST).
Complete Sun and Moon Data for One Day | aa.usno.navy.mil/data/docs/RS_OneDay.html
Obtain the times of sunrise, sunset, moonrise, moonset, transits of the Sun and Moon, and the beginning and end of civil twilight, along with information on the Moon's phase by specifying the date and location.
Daylight Saving Time | en.wikipedia.org/wiki/Daylight_saving_time
Daylight saving time (DST) origin, observation, rationales, criticism, trivia.
Greenwich Mean Time (GMT) | wwp.greenwichmeantime.com
Current Greenwich Mean Time and world times.
International Bureau of Weights and Measures (BIPM) | bipm.org/en/home
The task of the BIPM is to ensure world-wide uniformity of measurements and their traceability to the International System of Units (SI).
Military & Civilian Time Designations | wwp.greenwichmeantime.com/info/timezone.htm
From A - Z, a listing and description of the 25 integer World Time Zones from -12 through 0 (GMT) to +12.
Set Your Computer Clock Via the Internet | tf.nist.gov/service/its.htm
Allows users to synchronize computer clocks via the Internet. The time information provided by the service is directly traceable to UTC (NIST). The service responds to time requests from any Internet client in several formats including the DAYTIME, TIME, and NTP protocols.
The Official U.S. Time | time.gov
This web site is intended as a time-of-day service only. It should not be used to measure frequency or time interval, nor should it be used to establish traceability to NIST or the USNO.
Time and Date.com | timeanddate.com
Countdown counters, calendar, time zone and other travel tools, world clock - time zones.
Time Service Department | tycho.usno.navy.mil
The Official Source of Time for the Department of Defense (DoD) and for the Global Positioning System (GPS), and a Standard of Time for the United States.
Time Standards | scienceworld.wolfram.com/astronomy/Time.html
Information on c dynamical time (TDB), ephemeris time (ET), Greenwich apparent sidereal time (GAST), Greenwich mean sidereal time (GMST), international atomic time (TAI), mean solar time, and more.
Time Travel | pbs.org/wgbh/nova/time
Companion web site to the NOVA program "Time Travel," originally broadcast on October 12, 1999. In the program, leading physicists delve into the mystery of whether time travel is possible, and if so, how one might go about building a time machine.
Time Zone Converter | timezoneconverter.com
Find out the time anywhere in the world.
Time | en.wikipedia.org/wiki/Time
Just about everything you ever wanted to know about time, from Wikipedia.
When does Daylight Saving Time/Standard Time begin? | nist.gov/public_affairs/faqs/qdaylite.htm
Information from NIST the National Institute of Standards & Technology.
Worldtimeserver | worldtimeserver.com
Provides correct and current time in any world time zone, country or major city. Accurate adjustments for Daylight Saving Time (or Summer Time) are made according to each location's rules and laws.
WorldTimeZone | worldtimezone.com
Provides the world a free service and reference resource on global time, time maps, meeting planner / time zone tools.
Z-Time | maybeck.com/ztime
Article by Harold F. Maybeck, Meteorologist/ Lecturer - Natural Sciences Department, Plymouth State College, Plymouth, New Hampshire.