[R] julian

From: Rolf Turner <rolf_at_math.unb.ca>
Date: Sat 14 May 2005 - 22:23:24 EST

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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