REBOL [
Title: "Julian Calendar Converter"
Date: 8-Nov-1999
File: %julian.r
Author: "Russ Yost"
Purpose: {To provide conversions to/from julian day numbers/dates.}
Comment: {
(Note: REBOL provides day of year built-in, but technically
not Julian conversion.) I'm not sure where these
algorithms originated, but I used them in a
JForth program in 1992. The epoch date seems to be
that set by Joseph Scaliger in 1582, i.e., Jan 1 4713
BC, (that would be Julian day number 1), but as this
algorithm takes the average length of the solar
year as 365.2425, it works over the range 0001
through 3000. Beyond then, the discrepancy with the
true length, 365.242199, should lead to errors. (The
year 2000, being divisible by 400, will be a leap
year, although in general, years that are multiples
of 100 are not leap years. It *does* yield the
correct Julian Day number given in the 1999 World
Almanac (World Almanac Books, Mahwah NJ) pg 322, for
31 Dec 1998, 2,451,179.)
}
Email: rryost@home.com
library: [
level: 'intermediate
platform: none
type: 'tool
domain: none
tested-under: none
support: none
license: none
see-also: none
]
]
print {
Two functions are defined herein, date-to-jul, with argument a
date in format dd-mm-yyyy in year range 0001 through 3000, and
jul-to-date, with argument a julian day number (JD) based on 1 Jan
4713 BC, with 31 Dec 1998's JD = 2451179 (as given in the 1999
Almanac) and corresponding to the above mentioned range.
This latter function also provides the day of the week for
the given JD.
Some other useful functions:
diy, arg yyyy, yields the number of days in a given year.
febdays, arg yyyy, yields the number of days in February.
testj-d, arg yyyy, prints a yyyy calendar, 1 line per month.
}
jul0: 1720997 ; use with jul day based in 365.2425 days in year.
comment {There are 365.242199 solar days in a year, so the preceding
assumption will lead to trouble after year 3000, I think.}
comment { 'jul0 also compensates for extra days added in the calculation
of the number of days per month, and adjusts the result to agree
with 1999 Almanac's statement that JD of 31 Dec 1998 is
2,451,179.
}
daysinyear: 365.2425
daysinmo: 30.6001 ; a synthesized number that is used to calc
; days in each month.
weekdays: ["Sun" "Mon" "Tue" "Wed" "Thu" "Fri" "Sat"]
idiv: func [x y][to-integer ( divide x y) ]
comment {REBOL ought to include an integer divide function! }
comment { Th func 'dye calcs a Julian day no based on 1-1-0001, taking
into account years that are multiples of 4 are leap years
(Feb has 29 days instead of 28) unless they're multiples of
100 years, but those *are* leap years if they're multiples of
400.
}
comment {dye is day number of last day of year y }
dye: func[y][ (y * 365) + (((idiv y 4) - (idiv y 100)) + (idiv y 400))]
date-to-jul: func [date /local jul mo mox yr][
jul: date/day + jul0
yr: date/year
mo: date/month
either mo > 2 [mox: mo + 1 ] [
mox: mo + 13 yr: yr - 1 ]
jul: jul + (dye yr) + (to-integer mox * daysinmo)
]
jul-to-date: func [jl /local date da mo yr dano][
dano: jl - jul0
dow: remainder (dano + 6) 7 ; Sunday = 0
date: j2d jl
da: date/day mo: date/month yr: date/year
print [{date is } to-date reduce[da mo yr]]
print [{ day of week is }first skip weekdays dow]
]
j2d: func [ jl /local dano yr mo da dow mox][
dano: jl - jul0
yr: to-integer ((dano - 122.1) / daysinyear) ; print yr
mox: to-integer((dano - (to-integer(yr * daysinyear))) / daysinmo)
da: dano - (to-integer(yr * daysinyear))
da: da - (to-integer (mox * daysinmo))
either mox < 14 [mo: mox - 1][ mo: mox - 13]
if mo < 3 [yr: yr + 1]
return to-date reduce [da mo yr]
]
comment { jdye is JD of last day of year }
jdye: func [y][ date-to-jul (to-date reduce [31 12 y])]
comment {'diy yields the number of days in a year - useful for checking
leap years. }
diy: func [y][(jdye y )- (jdye (y - 1))]
febdays: func[y]
[(date-to-jul (to-date reduce[1 3 y])) - date-to-jul (to-date
reduce[1 2 y])]
comment {'testj-d prints a calendar, 1 line per month for its argument,
a year in format yyyy }
testj-d: func[yr /local strt-dt stjul dx da][
strt-dt: to-date reduce [1 1 yr]
stjul: date-to-jul strt-dt
print {}
for i 1 diy yr 1 [
dx: j2d ( stjul + (i - 1))
da: dx/day
either da = 1 [
prin [{^(line)} da {}]][
prin [da {}]]
]
print ""
]