Decimal Time - Time Zones

Standard time is defined by its use of time zones, which are one-hour offsets from UTC (GMT). Time zones are used to keep local civil time close to mean solar time. Decimal time standards use different time zones.

Universal Time

Coordinated Universal Time (UTC) is the international time scale from which standard time zones are derived. Due to variations in the rotation of the earth, leap-seconds are occassionally added to keep UTC within one second of Greenwich Mean Time (GMT). Uniform time scales, such as International Atomic Time (TAI) and Terrestrial Time (TT), are also historically based on GMT, but are not kept synchronized with the earth's rotation, and thus increasingly diverge from UT. Many representations of decimal time, such as Modified Julian Dates and astronomical calendar dates, use UT or uniform time scales.

Astronomical Time

Before 1925, astronomers kept time 12 hours behind civil time, so GMT started at noon at Greenwich, instead of midnight. This became named Greenwich Mean Astronomical Time (GMAT), or just astronomical time. Julian Dates are still based on astronomical time, in order to be consistent with older Julian Dates. Proponents of decimal Astronomical Time include jonathan jay, Lyle Zapato and Aristeo Canlas Fernando.

Central European Time

Swatch Internet Time uses Central European Time (CET), which is one hour ahead of UTC. Swatch claims that they are using "Biel Mean Time" based on the meridian passing through their headquarters in Biel, Switzerland, but Biel is only about 7.2 degrees east of Greenwich, more than a half-hour (22 .beats) behind CET. Their use of CET may be convenient in Europe, but conflicts with the fact that Universal Time is already the international standard.

French Decimal Time

French decimal time used local true, or apparent, solar time, that is, 5 o'clock on a decimal clock occurred exactly at true noon, when the sun reached its highest point in the sky at the location of the observer, and 10 o'clock was at true midnight, midway between two successive high points. Later, French civil time was based upon Paris Mean Time (temps moyen de Paris) until 1911, when it was changed to be temps moyen de Paris retardée de 9 minutes 21 secondes, which just happens to be the same as GMT. In the Republican Calendar, the new year was determined by the day on which the autumnal equinox occurred at the Meridian of Paris, which traverses the Observatory of Paris, according to true solar time.

Local Time

Excel serial dates are based on the local time zone settings of the computer's internal clock. Thus, they are usually synchronized with local civil time, even during Daylight Saving Time. Chronological Julian Dates correlate with whatever historical culture is being referred to. Any local time can be converted to decimal time of day simply by dividing the hours by 24, so 1:30 pm = 13.5/24 = 0.5625.

Decimal Reform

If decimal time of day ever becomes widely used for civil time, time zones would probably be changed, because 1-hour (0.0416 day) offsets, covering 15 degrees longitude, do not convert evenly into decimal fractions. If local times would continue to be used, they might be changed so that there are only ten time zones, each 0.1 day (36 degrees) apart, or perhaps twenty time zones, 0.05 day (18 degrees) apart, as with Aristean Time Zone.

Alternately, one time of day might be used everywhere, and individuals and organizations could choose to adjust their own schedules according to when the sun shines in their part of the world, without changing the time of day. For instance, someone in London might work from .400 to .700, someone in New York from .600 to .900, someone in Tokyo from .000 to .300, etc. In the summer, work times might be earlier, to take advantage of the extra sunlight, so in London one might work .350 to .650, in New York .550 to .850, etc. Thus, there would be no need for time zones or Daylight Saving Time, but people might keep track of when local sunrise occurs in different places, instead.

It has also been suggested that local standard time might continue in use, but that decimalized Universal Time could be represented alongside local times, which would clearly distinguish them. For example: 10:00 (.750).

If current local time zones are converted to decimal time of day, then the time zone will need to be noted along with the time. Currently, time zones are often designated by abbreviation, e.g. UTC, EST, PDT, CET, etc. However, since these abbreviations are defined by local law or convention and are not standardized, some time zones may have more than one name and abbreviation, some none, and in some cases the same abbreviation can be used for more than one time zone, so for these reasons it is often recommended that time zones be represented by their difference from Universal Time in hours and minutes. For instance, EST is -05:00, PDT is -07:00, Newfoundland Time is -03:30 and CET is +01:00. However, if these offsets were represented in decimal time, many of them would have a repeating decimal: -.04166, -.14583, etc. Fortunately, the military has assigned a letter of the alphabet to each of the 25 major time zones. UTC is designated Z for "zero" and pronounced "zulu" and Zulu time is designated by appending Z to the time, e.g. 00:00Z. Time zones east of Zulu are designated as A-M, and west of Zulu as N-Y, with J reserved for unspecified local time. These letters can be appended to the local decimal time of day, such as .750R in New York.

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