Julian Date Objects
The function jul is used to create jul (julian date)
objects, which are useful for date calculations.
as.jul and asJul coerce an object to class "jul", the
difference being that as.jul calls the constructor jul,
while asJul simply forces the class of its argument to be "jul"
without any checking as to whether or not it makes sense to do so.
is.jul tests whether an object inherits from class "jul".
jul(x, ...) ## S3 method for class 'Date' jul(x, ...) ## S3 method for class 'IDate' jul(x, ...) ## S3 method for class 'ti' jul(x, offset = 1, ...) ## S3 method for class 'yearmon' jul(x, offset = 0, ...) ## S3 method for class 'yearqtr' jul(x, offset = 0, ...) ## Default S3 method: jul(x, ...) as.jul(x, ...) asJul(x) is.jul(x)
x |
object to be tested ( |
... |
other args to be passed to the method called by the generic
function. |
offset |
For
|
The jul's for any pair of valid dates differ by the number of
days between them. R's Date class defines a Date as a number
of days elapsed since January 1, 1970, but jul uses the
encoding from the Numerical Recipes book, which has Jan 1, 1970
= 2440588, and the code for converting between ymd and jul
representations is a straightforward port of the code from that tome.
Adding an integer to, or subtracting an integer from a jul
results in another jul, and one jul can be subtracted
from another. Two jul's can also be compared with the
operators (==, !=, <. >, <=, >=).
The jul class implements methods for a number of generic
functions, including "[", as.Date, as.POSIXct,
as.POSIXlt, c, format, max,
min, print, rep, seq, ti,
time, ymd.
jul is a generic function with specialized methods to handle
Date and ti objects. A recent addition is a method to
handle IDate objects as defined in the data.table
package.
The default method (jul.default) deals with character x by
calling as.Date on it. Otherwise, it proceeds as follows:
If x is numeric, isYmd is used to see if it could be
yyyymmdd date, then isTime is called to see if x could
be a decimal time (a number between 1799 and 2200). If all else fails,
as.Date(x) is called to attempt to create a Date object
that can then be used to construct a jul.
is.jul returns TRUE or FALSE.
as.jul and asJul return objects with class "jul".
jul constructs a jul object like x.
jul with no arguments returns the jul for the current day.
The return value from asJul is not guaranteed to be a valid
jul object. For example, asJul("a") will not throw an
error, and it will return the string "a" with a class attribute "jul",
but that's not a valid julian date.
The Julian calendar adopted by the Roman Republic was not accurate with respect to the rotational position of the Earth around the sun. By 1582 it had drifted ten days off. To fix this, Pope Gregory XIII decreed that the day after October 4, 1582 would be October 15, and that thereafter, leap years would be omitted in years divisible by 100 but not divisible by 400. This modification became known as the Gregorian calendar. England and the colonies did not switch over until 1752, by which time the drift had worsened by another day, so that England had to skip over 11 days, rather than 10.
The algorithms used in jul2ymd and ymd2jul cut over at
the end of October 1582.
Press, W. H., Teukolsky, S. A., Vetterling, W. T., and Flannery, B. P. (1992). Numerical Recipes: The Art of Scientific Computing (Second Edition). Cambridge University Press.
dec31 <- jul(20041231)
jan30 <- jul("2005-1-30")
jan30 - dec31 ## 30
feb28 <- jan30 + 29
jul() ## current datePlease choose more modern alternatives, such as Google Chrome or Mozilla Firefox.