[R] julian
Rolf Turner
rolf at math.unb.ca
Sat May 14 14:23:24 CEST 2005
I have a problem wherein I need to convert back and forth from
dates of the form "2000-04-11" to Julian dates. After some
experimentation I found that
> z <- strptime("2000-62",format="%Y-%j")
> z
gave me "2000-03-02" --- i.e. March 2 which is as it should be
according to my (not too reliable) arithmetic.
Going the other direction, more experimentation led me to
> julian(z,origin=as.POSIXct("1999-12-31"))
which gives
Time difference of 62 days
and I thought that I had a working (if not fully comprehended)
syntax. My illusions were shattered when I tried another Julian
date, 102:
> z <- strptime("2000-102",format="%Y-%j")
> z
gives "2000-04-11" --- i.e. April 11, which checks with what I think
it should be. But reversing the direction:
> julian(z,origin=as.POSIXct("1999-12-31"))
gives
Time difference of 101.9583 days
Where did the missing 0.0417 days go to?
What syntax should I really be using?
Thanks.
cheers,
Rolf Turner
P. S. I'm still on R version 2.0.1 (Linux) if that's of any relevance.
R. T.
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