[R] Convert date to decimal days

[Prof Brian Ripley]

I think you will find that the 'catastrophic loss of significance' is in 
the printing at the default level.  Try print(x, digits=15).

On Sun, 3 Aug 2008, cls59 wrote:

>
> Hello all,
>
> I have a quick question about formatting date strings..

Datetime strings, methinks.  If you do just want days, use as.Date not 
as.POSIXct.

> I am currently debugging some Matlab code someone else wrote and since it is
> so bad that I have to go through it line by line I figured that I would just
> rewrite the thing in R.
>
> The code produces plots of wave spectra with decimal days since the Epoch as
> the x axis and wave period as the Y axis. I am able to convert the date
> stamp in the input file to a POSIXct object with the following call:
>
> date = as.POSIXct(header[[1]][2],tz='GMT',format='%Y%m%d_%k')
> date
> "2008-07-11 03:00:00 GMT"
>
> So every thing looks groovy, using as.double(date) gives me the number of
> seconds since the epoch which reproduces the correct date when fed to date
> -u -r on the command line.
>
> However, I can't seem to find a clean way of converting the date to decimal
> days since the epoch. Using
>
> as.double(date)/86400
>
> Gives:
>
> 14071.12
>
> Which is a catastrophic loss of significance, the number should be 14071.125
>
> I tried playing around with the format command to see if I could get R to
> format the date differently, but no luck.
>
> Any suggestions?
>
> -Charlie
> -- 
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595