[R] Julian Dates

[Gabor Grothendieck]

What you can do to handle this timezone problem is either to use
POSIXt with GMT or use chron (which does not use timezones so 
can't cause problems like this):

Suppose:

SLDATX <- c( "1986-01-06", "1986-01-17", "1986-02-02", "1986-02-04",
,"1986-02-04", "1986-02-21", "1986-03-06", "1986-03-25",
,"1986-04-06", "1986-04-10" )

# then using POSIXt in the GMT timezone:

TIMESOLD <- as.numeric( julian( as.POSIXlt( SLDATX, tz="GMT" ),
  origin = as.POSIXct( "1986-01-01", tz = "GMT" ) ) )

# or the alternative using chron:

require(chron)
TIMESOLD2 <- as.numeric( chron( SLDATX, format="y-m-d", 
  origin = c( month = 1, day = 1, year = 1986 ) ) )

all.equal(TIMESOLD,TIMESOLD2)

---
Date: Mon, 15 Dec 2003 22:37:59 +0100 
From: Uwe Ligges <ligges at statistik.uni-dortmund.de>
To: Ko-Kang Kevin Wang <kwan022 at stat.auckland.ac.nz> 
Cc: R Help <r-help at stat.math.ethz.ch> 
Subject: Re: [R] Julian Dates 

 
 


Ko-Kang Kevin Wang wrote:
> 
> Hi,
> 
> I'm a bit confused how julian() works. If I understand right, it returns
> the number of days since the origin.
> 
> I have a vector:
> > SLDATX[1:10]
> [1] "1986-01-06" "1986-01-17" "1986-02-02" "1986-02-04"
> [5] "1986-02-04" "1986-02-21" "1986-03-06" "1986-03-25"
> [9] "1986-04-06" "1986-04-10"
> 
> And when I did:
> > TIMESOLD <- as.numeric(julian(as.POSIXlt(SLDATX),
> + origin = as.POSIXct("1986-01-01", "")))
> 
> I got:
> > TIMESOLD[1:10]
> [1] 5.00000 16.00000 32.00000 34.00000 34.00000 51.00000
> [7] 64.04167 83.04167 95.04167 99.04167
>
> THe first 6 values from TIMESOLD is obvious, however I'm not sure why I
> got decimals from the 7th value, as my input vector does not have any
> specific "times" after the dates.

That's because of the timezone you are using (winter/summertime).
Look what the difference of 0.04167 really is: 0.04167 * 24 ~ 1
hour.....

Uwe Ligges

> Any insights would be greatly appreciated...;-D
> 
> --
> Cheers,
> 
> Kevin
> 
> ---------------------------------------------------------------
> "Try not. Do, do! Or do not. There is no try"
> Jedi Master Yoda
> 
> ----
> Ko-Kang Kevin Wang, MSc
> SLC STATS 10x Workshop Coordinator
> University of Auckland
> New Zealand
> Homepage: http://www.stat.auckland.ac.nz/~kwan022
> Ph: 373-7599 x88475
> 
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