Time, Mr. Spike, is what turns kittens into cats

There are two basic ways to look at time: calendar time and elapsed time. Calendars are fundamentally attached to and reflective of the position of the Earth and/or Moon in their orbits. Elapsed time is the linear measure of time, such as that measured by atomic clocks, or a stopwatch. Date is an always approximate measurement of a physical process (the Earth orbiting the Sun, the Moon orbiting the Earth), while duration is a measured quantity (the number of seconds passed from a given point). We use duration, in the form of clocks and watches, to help us keep track of the date. Since the two are fundamentally different views of time, reconciling them is difficult at best, and always approximate.

Date-times are based on the Gregorian Calendar. The written form of dates and times is specified in ISO 8601 as YYYY-MM-DD hh:mm:ss-/+hhmm (for example, 2004-10-22 16:22:55-0700). The time zone notation (-0700 in this example) can be abbreviated 'z' or UTC if it is 0000, but other time zones are best noted as the number of hours and minutes from UTC, since there are no consistent standards for abbreviations.

Universal Time Coordinated (UTC)

Clocks assume that each day is the same length and has a whole number of seconds (86400) in it. In fact, the length of the day varies throughout the year, so we use the convention of a mean solar day for our clocks. This is close but not exact because the day also becomes slightly longer over time. (Currently a msd is approximately 86400.002 seconds long.) To account for this discrepancy leap seconds are added (or subtracted) when needed, based on astronomical observations. This currently takes the form of adding a second every 500 days or so, when the difference between astronomically calculated time and UTC is different by more the .9 seconds. The second is added at 23:59:59 on December 31 or June 30. In this way clocks (which measure the passage of time by adding up seconds) stay in sync with the Earth's rotation.

Atomic Time (TAI)

International Atomic Time, or TAI, was chosen to coincide with Universal Time at midnight on 1958-01-01 (julian date 2436204.5): this is the start of TAI, and it should be emphasized that it makes no sense to refer to a date prior to this in TAI. Unlike other time measurements, TAI is a linear count of seconds: there are no leap seconds. Thus, there is as of 2003-01-01 a 32 second difference between UTC and TAI; i.e., 32 seconds have been added to UTC to keep it synchronized with the Earth's rotation.

Julian Day Count

Julian day count is a count of days from an arbitrary historical point in time (1 January 4317 BC, Julian calendar). Thus no leap days are needed, because there is no correlation to the solar year. It is used largely in the context of astronomy, as an unambiguous way of noting a particular date and time, and one which easily enables the calculation of days passed without having to deal with leapdays and other calendar vagaries.

The time-of-day component of the julian day count is usually just a decimal conversion of IDT (International Dateline Time, or UTC plus 12 hours). As far as i know, it uses UTC, not TAI time; i.e., leap seconds are included. The home page of this site shows a julian day count using IDT.

Clock Reform & Metric Time

So-called Metric Time is usually in fact some form of decimal time. The two are not the same: the metric unit of time is the second, with minutes, hours and days as recognized units defined in seconds. This is not a decimal system, however, unlike most of the rest of the metric system. Nor can it be coerced into a decimal form — there are by definition 86 400 seconds in a mean solar day, an unwieldy number from a decimal perspective.

There are at least two different approaches one could take to creating a decimal time. The most frequent one, and the one used in julian days, represents the time of day as a decimal fraction of the day. Each 1/100 000 of a day is .864 seconds long, making it a convenient unit; one hundred of these units is 1.5 minutes, and one hundred of those is about 2.4 hours. For convenience then 12:00:00 becomes 5:00:00, or .500 00. Various systems using this method have been proposed, some based on other time zones (Swatch time is UTC+0100, for example). Clearly, in order to conveniently use this method of time keeping adjusted to local time, the number of time zones would have to change.

One of the advantages of such a system is that it could be implemented anywhere. For example, the msd on Mars is approximately 24h 39.6m long, which would make a standard second-clock difficult to use; but a decimal clock just needs to be adjusted so that it counts in 887.76 ms units and it will look just like an Earth-based clock. The time of .5, or 5:00:00 is still midday, and the clock functions as it is intended — not to measure duration, but to track the time of day.

Another approach would be to simply count seconds, ignoring days, weeks, months and years entirely; i.e., making no attempt to reconcile duration with date. This is done, after a fashion, with atomic time. Computers do this as a matter of course. Most Unix-based computers count the number of seconds from 1970-01-01, and the RFC 868 standard defines network time as seconds from 1900-01-01 00:00:00 UTC. The computer then converts this number into a date for the user. Because of the type of number used to calculate this time, it will not last beyond 2036. Newer computers will have corrected this by then, of course :-)