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AAT ICAS 2040 ✧ guidelines for scale calculation

document note: this document is part of the Integrated Chronological Applications System (ICAS). Alliance for the Advancement of Technology (AAT) provides ICAS standards documents subject to terms of use described in document AAT ICAS 9010. please refer to other key AAT ICAS standards documents accessible via the AAT ICAS web site at for important information about ICAS.


AAT ICAS guidelines for scale factor calculation

this document describes AAT ICAS guidelines for scale factor calculation. please refer to the following documents for additional calculation guidelines:


AAT ICAS policy on calculation methods

AAT ICAS considers that particular methods for selecting and using calendar and clock formats, and for determining calculation factors relating to the designation and observance of calendar and clock dates and times should be reserved to users and licensees. AAT however encourages the sufficient representation of any particular method of formatting or calculation. for additional information about the calculation factors used by AAT, please refer to the AAT metrication policy (AAT ICAS 9030).


overview of world calendaring

a Julian calendar of 365 days and 6 hours in a year (365 days and a leap day every 4 years) was introduced in AUC 709 (BC 45). the AD historical era was introduced during the sixth century AD. (Dionysius Exiguus established year Anno Domini 532 to correspond to year 248 of the Anno Diocletiani era.) the BC historical era was introduced during the eighth century AD. (English historian Bede established practice of counting backward from AD 1.) the modern Gregorian calendar was introduced in 1582 October to conform the Julian calendar to calendar dates for solstice and equinox. various transitions from the Julian calendar to the Gregorian calendar recalculated the formula used for determining leap years, introduced a tropical calendar correction (Julian calendar date 1582 October 04 Thursday was followed on the next day by Gregorian calendar date 1582 October 15 Friday), and applied a change in the beginning of a new year from March 25 to January 01. these transitions occurred in different regions from AD 1582 to 1923. the Gregorian calendar was adopted by England and its colonies in AD 1752, and is presently used by most countries. astronomers also use a system of Julian dating counting from Julian Day 0, which began at noon UT on BC 4713 January 01. Hebrew and Muslim calendars are based on a lunar cycle.


overview of world timekeeping

modern timekeeping is often reckoned in terms of a traditional system of 24 hours per day, with 60 minutes per hour, and 60 seconds per minute. the 12-hour clock dial is likely derived from traditional methods of timekeeping based on daylight.

methods of modern timekeeping vary by region and organization, and include a 24-hour dial (00:00–23:59), a 12-hour dial repeated twice a day (12:00–11:59 am, 12:00–11:59 pm), and a decimal representation of 1 day. different methods for decimalizing time in terms of either the hour, minute, or second are also used among various organizations for particular purposes. methods for decimalizing time in terms of a main unit of 1 day are used in astronomy, computer science, multimedia development, space aeronautics, and international standards.

a world standard time of international time zones was established in 1884. Coordinated Universal Time (UTC) was established for civil timekeeping in 1972.

as timekeeping technologies have developed, modern timekeeping systems now reference atomic frequencies by which practical measures of time such as the second and the tikochron are now fixed to high levels of accuracy.

top document updated:

system identifiersdatetime
Uniform CalendarUCUCN 12019 S19 Yellow
Inter-Dial ClockIDCzone(UT)t339 tt050
'ICAS in use' can accommodate calendar and clock formatting 'for all people, for all time'.
day of yearD-o-YAD common year day139
Gregorian calendarGG2019 May 19 Sunday
seconds, minutes, hoursSMHUT08:08:14
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