Rata Die (R.D.) is a system for assigning numbers to calendar days (optionally with time of day), independent of any calendar, for the purposes of
calendrical calculations. It was named (after the
Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
ablative
In grammar, the ablative case (pronounced ; sometimes abbreviated ) is a grammatical case for nouns, pronouns, and adjectives in the grammars of various languages; it is sometimes used to express motion away from something, among other uses. ...
feminine singular for "from a
fixed date
Date or dates may refer to:
*Date (fruit), the fruit of the date palm (''Phoenix dactylifera'')
Social activity
*Dating, a form of courtship involving social activity, with the aim of assessing a potential partner
** Group dating
*Play date, a ...
") by Howard Jacobson.
[It was called ''absolute date'' i]
GNU Emacs
Rata Die is somewhat similar to
Julian Dates (JD), in that the values are plain
real number
In mathematics, a real number is a number that can be used to measure a ''continuous'' one-dimensional quantity such as a distance, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small variations. Every ...
s that increase by 1 each day. The systems differ principally in that JD takes on a particular value at a particular absolute time, and is the same in all contexts, whereas R.D. values may be relative to
time zone
A time zone is an area which observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it ...
, depending on the implementation. This makes R.D. more suitable for work on calendar dates, whereas JD is more suitable for work on time per se. The systems also differ trivially by having different epochs: R.D. is 1 at midnight (00:00) local time on January 1, AD 1 in the
proleptic Gregorian calendar
The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to the dates preceding its official introduction in 1582. In nations that adopted the Gregorian calendar after its official and first introduction, dates occ ...
, JD is 0 at noon (12:00)
Universal Time
Universal Time (UT or UT1) is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle wit ...
on January 1, 4713 BC in the
proleptic Julian calendar
The proleptic Julian calendar is produced by extending the Julian calendar backwards to dates preceding AD 8 when the quadrennial leap year stabilized. The leap years that were actually observed between the implementation of the Julian calendar in ...
.
Forms
There are three distinct forms of R.D., heretofore defined using Julian Dates.
Dershowitz and Reingold do not explicitly distinguish between these three forms, using the abbreviation "R.D." for all of them.
Dershowitz and Reingold do not say that the RD is based on Greenwich time, but page 10 state that an R.D. with a decimal fraction is called a moment, with the function moment-from-jd taking the floating point R.D. as an argument and returns the argument -1721424.5. Consequently, there is no requirement or opportunity to supply a time zone offset.
Fractional days
The first form of R.D. is a continuously-increasing fractional number, taking integer values at midnight local time. It is defined as:
:RD = JD − 1,721,424.5
Midnight local time on December 31, year 0 (1 BC) in the
proleptic Gregorian calendar
The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to the dates preceding its official introduction in 1582. In nations that adopted the Gregorian calendar after its official and first introduction, dates occ ...
corresponds to Julian Date 1,721,424.5 and hence RD 0.
Day Number
In the second form, R.D. is an integer that labels an entire day, from midnight to midnight local time. This is the result of rounding the first form of R.D. downwards (towards negative infinity). It is the same as the relation between Julian Date and Julian Day Number (JDN). Thus:
:RD = floor( JD − 1,721,424.5 )
Noon Number
In the third form, the R.D. is an integer labeling noon time, and incapable of labeling any other time of day. This is defined as
:RD = JD − 1,721,425
where the R.D. value must be an integer, thus constraining the choice of JD. This form of R.D. is used by Dershowitz and Reingold for conversion of calendar dates between calendars that separate days on different boundaries.
See also
*
Decimal time#Fractional days
*
Julian date
The Julian day is the continuous count of days since the beginning of the Julian period, and is used primarily by astronomers, and in software for easily calculating elapsed days between two events (e.g. food production date and sell by date).
...
*
Lilian date
References
{{reflist
Applied mathematics
Calendars
Calendaring standards