[R] Converting the time in a numeric value

[Prof Brian Ripley]

On Sun, 10 Jul 2011, Amy Ruiz Goyco wrote:

> Hello:

> I am new using R.  I have a file that contain in the same columns 
> date and time like for example 2011/10/03 12:34:45.123423 p.m., but 
> when I read the file and display the vector, I see of this way 
> "2011-10-03 12:34:45.123423".  I need to convert the time in a 
> numeric and the date if is possible, but I don't need this to 
> compute.  Thus, I used this tiempo=substr(time,12,26) to selected 
> the data that I need, but I don't know how I can change this to a 
> numeric values.

You need to clarify what you mean by 'date' and 'numeric' here. At a 
guess, 'numeric' might mean 'number of seconds past midnight'.  We 
can't even do that, since we don't know the timezone (and it differs 
on DST transition days).  So you need to read a well-informed article 
on date-times (not the un-refereed one in R-News 4/1) to gain more 
understanding.

Also, your AM/PM indicator is very non-standard.

But here are some pieces for you to work with

dt <- "2011/10/03 12:34:45.123423 p.m."
t0 <- substr(dt, 12, 26)
PM <- substr(dt, 28, 31)

date <- strptime(dt, "%Y/%m/%d")
t1 <- unclass(as.POSIXct(paste("1970-01-01", t0)))
time <- ifelse(PM == "p.m.", t1+12*3600, t1)

> date
[1] "2011-10-03"
> print(time, digits=12)
[1] 84885.123423


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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)
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